Thanks for the replies, everyone.
FujiFlyer - re: possible PSU 'brownouts', I guess that's a definite possibility, the PSU was running very hot. I can't figure out if the PSU fan was deliberately installed to suck through the PSU, or if it was a mistake at the factory. The fan is on the bottom of the PSU and was sucking air through the back of the case, via the PSU, and blowing it onto the CPU area of the motherboard - I guess this might have been deliberate??
In any case, I've now turned the fan over so that it blows through the PSU and it's running a lot cooler.
The thing that makes me suspect that the lockups were due to CPU overheating rather than PSU overheating was that the only change I made was to swap out the CPU fan - this may well have had an indirect effect on the PSU temperature, but it must surely have had a much larger effect on the CPU's temperature?
I'm happy now though - it runs without problems, so that's pretty much that. I might try some 'proper' thermal compound next time I've got the case off, but other than that I'll just keep an eye on the temps... :o
Cheers for the help.
cbl.