Aspire 6920G shutting down

Hello everybody.
A weird problem appeared on my brother's 6920G. Well, all started when the graphics card (9500m) went off. I ordered and installed an HD3650 256mb DDR3 (MXM2).
I installed the new card, cleaned thoroughly the fan and heatsinks, applying fresh thermal paste. The laptop booted instantly, loaded windows and after 2 minutes
shut down. I instantly thought that was an overheating issue, though (MARK THIS) it wasn't hot at all...I then realised that the fan would't work at all.
So, next step - ordered a new and original fan. Installed straight forward and the fan worked like a charm, but the shut down happened again. I even changed the cpu,
same type (T8300) , no good... There is no overheating, but the way it shuts down (from my experience) is the same as a laptop overheating or ac charger is bad.
The shutdown appears even if I enter the BIOS and leave it there running, so it isn't a windows/driver thingy. I tried a new ac cable from another 6920g, updated BIOS, no luck.
Of course I have tried several ac power supplies, Original acer... Nothing seems broken, I mean none of the small componets seem faulty ( diodes,capacitors etc).
Any thoughts people? It's driving me crazy, can't figure out any possible reason except the motherboard itself. I am experienced at soldering and such stuff, any guidelines I could try?
Mark a last thing: When I enter the BIOS running ONLY on battery, then plug in the charger, the screen goes a bit dimmer, I dont know for sure if this is normal,
or if this is any clue of something's wrong with the circuitry. but the shutdown happens no matter if I use only the battery or the charger alone.
Any help/idea would be greatly appreciated, please forgive any grammar mistake, as english isn't my mother language. Well, let's have this figured out, I'm willing to try anything, since the notebook is unusable.
Thank you in advance.

 

Any thoughts people?

 

You seem to have excluded pretty much every possibility apart from the worst one which would be a mobo failure.

I had a case like that a while ago when a chipset that was somehow glued to the motherboard would start to separate as the notebook got hotter. As a result the glue got softer and it wouldn't make hold the chipset firmly in place anymore. It started happening after good two or three years of operation.

It sort of looks like a similar case. I tested it on a mobo alone (dismantled laptop fully) with only CPU and RAM in place. Heating up the chipset (with a hair dryer) and pressing it into a mobo would help for a while. I ended up replacing the mobo though.

Did you check other RAM modules?

 

The HD3650 has a thinner GPU core than the original card, so a copper shim is often needed on the MXM card. With simple thermal paste, the cooler won't touch the core - as a result, the cooler will stay cool to touch, while the GPU overheats and shuts the system down.

 

you heard the man either get copper shims, or get an another nvidia card. I ehards the ati 3000x series are quite warm. the 9600m gt nvidia ddr2/3 are not as bad

 

Well thank you for the replies, but the issue is not caused of overheating. The system may shutdown after 1 minute, and the vga is still quite cool itself.
I was wondering if this problem has anything to do with this: Toshiba Satellite - Locks up when plugged in - Badcaps Forums

 

I really think it could be the vga though the only way to test this is to get another one. >.>