ok, i'm now certain its the processor that's heating. got speedfan to work and it reported some fairly high temperatures. cpu got up into the 60's (celcius) and "core" temp at one point got to 118!!! which would explain why the thing is cutting out.
i haven't had chance to open the case yet but what should i be looking for when i do. all of the fans are working ( 1*front and 1*side case fans plus the cpu fan) so i dont think that's it. could it be a thermal paste problem, or should i upgrade the cpu fan ( at the moment it a smallish thing that runs contstantly at just under 3000 rpm)