I have a Thinkpad T42 and the gpu (Mobility Radeon 7500) started overheating yesterday, I'm almost certain this is the problem as when I put a small heatsink over it the laptop was functioning as normal (but I want to use it with the case on)

This model has a heatsink just over the cpu, but models with a 9000 and higher gpu's have an extended one over the graphics chip so I'm going to get one of them.

Symptoms: runs fine for about 5 minutes then freezes and screen corrupts, then after reboot black screen or screen not on at all and fan on full to cool the overheated chips.

Would like to know what else it could be or why its only just started overheating as I've had it for a while now no problems and its completely dust free, not overclocked, temperature readings for gpu are 40-45C @idle which isn't overkill + ibm/lenovo don't seem to think it even needs a heatsink on the radeon chip. :confused:

I have a T60P that's about 2 years old and have a similar problem. I found a useful system temperature monitor at:
TPMonitor that let me argue with Lenovo's tech support and got them to replace my cooling fan. It failed about 6 months into my owning my laptop. I've read many other posts about Lenovo's having this same problem so my only other suggestion is to try to get them fix your problem or look to join some class action lawsuit against them.