Okay last night on the advice of HG I removed the RAM. I left one stick in, and it seems okay. It restarts and turns on straight away (no monitor trouble) and CPU usage is between 2-5%. Could it all have been caused by the RAM though?

I'll try with both sticks in when I get in. The other stick seems to put CPU usage at 100% when I put it in but that could just be chance; ie something else decided to choke the CPU while that RAM was in.

Anyway hopefully its fixed now but I would like to have 1GB of RAM again instead of 512MB