Poor Gaming Performance in XP, SR MBP

This issue has perplexed me for the past couple of days so I'm wondering if anybody here will be able to help me out with it.
My Macbook Pro is the 2.2ghz Santa Rosa version with the 128mb DDR3 8600M-GT.
Up till just recently my Macbook Pro has been great when it comes to gaming in XP, running everything I've thrown at it without breaking a sweat, no troubles whatsoever.
One night the other week I was playing a bit of Left4Dead when I noticed the game was being a fair bit laggier than usual, seemingly in patches where it was performing just fine and patches where I was getting pretty low framerates. I didn't think too much of it at the time, and figured that maybe a L4D update had upset it or something, and that it wouldn't be a bad idea to go get the latest video drivers the next day, as the ones I was using at the time were at least 6 months old. So I did just that and got the latest WHQL certified drivers from Laptopvideo2go, and installed them to find that my framerate was not improving the next time I played. Rather, it was consistently laggy, rather than just in patches. I tried a spot of Team Fortress 2 and Test Drive Unlimited to see if it was just Left4Dead or what, but they were both significantly more laggy than usual, especially Test Drive, which is pretty much completely unplayable now. I noticed that if I rebooted it, there was a chance it would boot up and my gaming performance would be completely back to normal, not laggy at all, so a couple of times I tried that, and after two or three reboots, then loading up the game to see what the performance was like, it seemed to click and work properly for whatever reason. Eventually I lost my patience with it and reformatted my windows partition with a fresh install of XP SP2, then installed the laptopvideo2go drivers again, along with Test Drive Unlimited to test. Due to my internet being speedcapped at the time, I was still using whatever Boot Camp drivers were on the Leopard disk, rather than the latest ones, and annoyingly my performance had not been fixed by formatting. Once my speedcap had expired, I downloaded XP Service Pack 3, Boot Camp 2.1 driver update plus letting Windows Update update me right back up to date. I also noticed that nVidia had finally decided to release proper drivers for mobile cards (rather than having to get the regular drivers and mod the INF file, which has always worked fine for me in the past), so I got the newly-released official mobile nVidia drivers and tried them. So with the Windows partition completely up to date with the latest drivers for everything. and all games patched up to latest spec, it still is not working properly.
Still getting the really low framerates, especially in TDU and L4D. Last night I must have rebooted 5 or 6 times, and each time it was the same, lagggg. This morning I booted it up cold straight into Windows and my gaming performance was just fine again, no lag, running perfectly. I went into OSX to check my emails and whatnot, booted back into Windows and my performance was once again really slow.

So, after all that, can anyone offer any ideas?
Keeping in mind that it is coming into summer here, yesterday was quite a hot day, I figured that heat might have had something to do with it, but I checked with GPU-Z and CPU-Z that my GPU and CPU aren't throttling down whilst gaming and causing the lag, they're both running at full speed, as they should be. The heat thing would explain why it worked fine coming off a cold boot rather than going into Windows after being in OSX for a few hours and the notebook being all warmed up from that.
Another theory of mine is something to do with power profiles, maybe Windows thinks that it is running unplugged, and slowing performance, despite it not actually being? I noticed that performance does not get any worse when you unplug it and try on battery when its all laggy to begin with.
Aside from those ideas, I'm spent. I've no idea whats going on with it, but I do not think that my 8600GT is on the way out, as any games in OSX still perform as well as they always have done. Nor have I experienced any artefacts or scrambling or any other of the symptoms of a failing GPU.
So, any ideas would be very helpful.
Thanks!

 

what kind of framerates and settings are we talking?

net_graph 1 in the console for source games

 

in windows server 2008 (vista x64 minus bloat) I get 60 fps in left4dead with shaders at med and all other settings at high, with 2x aa and 4x af in 1440x900.

dips to 35 though if assaulted by the horde.

im using the nvidia laptop drivers from the site, and the 2.1 bootcamp drivers for x64.

 

I tested L4D when it was working properly last night, was getting a good 40-60fps depending on hordes and whatnot, but it never dropped below 40, very smooth. 1440x900, shaders on med, everything else high, no AA or AF.
When TDU is working properly it is probably around 25-30fps, running fairly nicely. When its playing up it dips to around 4-5 fps, which is very very annoying.
L4D I haven't tested when its playing up, but it is quite a noticeable reduction in framerate, it'd have to be under 20fps.

 

Can anybody help?

 

When Left 4 Dead is running, hit CTRL-ALT-DEL and go to processes. Right click left4dead.exe and click set affinity. Uncheck all CPUs except CPU 0. Should fix it.

 

Disable Multi-core rendering in L4D. It's bugged and it's causing several freezes and problems to a lot of people

 

Agree, but this problem is very funny, I have it on my desktop with Q6600 + GTX 280. But on my SR MBP, it doesn't happend. Disabling Multicore Rendering will affect the performance quite badly. My MBP can run L4D @ 1440x900 All high (except medium Shader) @ 40+ FPS with MR on, but with MR off, it easily falls down to 2x FPS.

PS: Anybody got good tweaking tips for L4D on MBP ?

 

You're not overheating or anything are you? Sometimes the GPU will downclock if it gets too hot and, if this happens, you will get low framerates.

Edit: Sorry, just seen the last part of your post and you said it was not downclocking.

Very strange.