Hi all - i think i have a clearer picture of this issue now. i applied some more thermal paste to the heat sink and removed a couple of cards and got the machine to boot. I then fired up the sensor modules and drove the fans harder, and the system sat idling ok at around 47C. Then I added the cards back in and the system idled at about 55C. Whenever the PC works harder (i mean web browsing, not audio processing) the temperature shoots up to 64C and the fans struggle to get the temp down. This is all with the case open! I'm thinking the system is running too hot :S
The system lives in this antec rackmount unit, which has disappointed me from day 1. I had to remove a chassis fan to fit the cpu fan (my akasa heat pipe or even the standard intel jobbie), and the psu fan doesn't turn at all! This means half the expected intake/outtake isn't even happening, which may explain the overheating. I think it's time to get a new case! Any advice is greatly appreciated :)