Vaio SR hanging when using battery power

I have a Sony Vaio VGN-SR19VN - (UK spec) 4gb ram / P8400 @ 2.26GHz / ATI Radeon HD 3450, brought around November 2008.

I seem to be having issues with running on battery power where it just randomly freezes whilst i.e. browsing (with any of the browsers - chrome, firefox, IE7), eclipse IDE, process explorer, with or without XAMPP (apache/php/mysql). When running with the AC adapter plugged in, it will happily run all day and night without hanging.

Away from mains power, there's a random chance of it hanging - I can go through several sessions of battery running down / switch to AC / battery without any issues; on the other hand it has frozen a minute after being unplugged onto battery mode (I use freezing to mean where the screen nor the keyboard status lights reacts to anything - USB mouse / trackpad; Windows is stuck on the same frame - like viewing a snapshot).

I have stopped RM Clock from running (I had originally undervolted the processor and ran numbers of tests at the time before I settled on the results - that had worked solidly for months), disabled startup items, uninstalled the preinstalled McAffee antivirus/firewall, but using the system still causes it to freeze every so often.

Curiously, there have been many times when it freezes whilst I am scrolling a document or browser with either the trackpad or a USB mouse, and there are times when it freezes when windows are open, but no one is using it.

The battery itself seems to be fine, as when I do manage to run it without freezing, it can, and does, last for 3 hours before I need to hibernate it or plug it back in.

Now, this is driving me nuts, as 1) it doesn't have a consistant pattern as to when it freezes; so 2) I cannot work out what not to do, and 3) it means I cannot use this laptop with confidence for on-the-road work as I brought it for. I have already lost work performed (but unsaved) due to this.

Just for testing, I am currently running Othos with small FFTs to stress the CPU - 20 minutes in so far.

The only way to recover from this is to do a forced power off (holding down the power button), otherwise it will happily display the current screen state until the battery runs out - or forever like this when plugged back into the wall.

Any probable cause here before I defer to Sony support? Considering it runs fine under mains power, I would have presumed something with the processor not liking the state it is in under battery load, but this being inconsistant anyway, and with RM clock being disabled (hence it is as it was when I received it), I am left with what is essentially a desktop bound PC.