IBM NetVista POST problems

Hope somebody can give me advice.

Rebuilt an IBM M42 8304-KGT. All working perfectly and ran all the PC Doctor 5 tests yesterday followed by DXDIAG.

Yesterday afternoon reinstalled Nero 6 (couldn't find the key until then). Still no problems.

Then installed the inf file and ICC profile for the LCD TFT monitor (IBM 9297) and still all OK. Shut down and left until today.

This morning booted up and a keyboard error appeared on POST. All three lights were on on the keyboard.

I powered off and checked the PS/2 was in correctly. I took it out and put back in again.

Powered back on and now there was a message checking the extended RAM. However as soon as it gets to 2048 MB (correct amount) the screen goes to 'no signal) and from then on the LCD TFT flashes as though it were receiving no signal and the base unit just sits. I had no idea if is doing anything!

I've tried booting from a DOS version of PC Doctor but of course as I can't get past POST I can't boot from even a floppy drive. I've also tried resetting the BIOS to defaults, doing the CMOS reset as prescribed on the IBM website and replacing the CMOS battery. I've checked to ensure the wireless PCI card and RAM is firmly connected and the RAM is recognised correctly.

Everything was perfectly fine yesterday so I don't really know what has happened. Windows XP is SP3 with all updates. The PC has AVG free 8.5 with all current updates as of yesterday.

I'm not really a hardware person so don't know what to check next.

Anybody give me any pointers?