Sounds like your system may be overheating.

Try running other 3D programs that uses the CPU to see if it locks up.

Concur with Edgar. Ultimately, it comes down to overheating. Download MotherBoardMonitor or other utility which gives you the CPU and motherboard temp.

Might consider a different CPU fan that has more cooling. I love Zalman.

Thought so too, but although the cpu gets warm, 45C, it doesn't reach frying temp. Mobo even lower (as expected). Will look again tomorrow after some deep thought... (its past midnight here now and anyone who knows me will confirm that I need my beauty sleep.)

It does sound like classic overheating symtoms, but maybe the graphics chip, not the CPU.

GPU running at 63-65C under most 'heavy' conditions. I'll try a big force-cooler to see if I can drop the temp further, but it is rated over 100C (120C+). I might even risk some freezer spray via the heatsink if I can get away without cracking the chip!!!

Don't use freezer spray. You'll crack the chip. And that is hot. Mine runs at 23c to 30c. But then again I'm water cooled. Do you have your case moded? I know some people have lights in there case that will heat up the case if you don't have enough fans in the case. What kind of fans do you have in your case and how many? And what kind of case do you have? This will all help us to help you.

Thanks for the thoughts guys. I stuck a cold (2deg C) airstream onto the cpu and gpu in turn, and gave them as long as necessary, but the hang still hit after about 2 hours approx in both cases. Then a thought struck me about the difference between the US English and International English versions of the drivers. After a spot of googling, general opinion seemed to favour older versions of the driver. I've installed/ tested/ uninstalled the latest versions of the drivers (91.47_forceware_winxp2k_international_whql) then (91.47_forceware_winxp2k_english_whql) then 91.36 then 84.21 then 81.98..... and with some help from 3Dguru.com finally settled on 76.44 which is described by many as being the last stable bugfree version. And it seems (so far) to have worked. No hangs, hiccups, etc. The temperatures seen to be as before, but now I have unchecked the "Limit frame rate" box and it still seems to be working. There has been another bonus too... there is a level of detail in the Red Lionfish that I didn't even know existed. Jim, deep respect for the quality and rendition!!

Amazing. Glad things worked out for you and will make sure to remember 76.44 is the weapon of choice...