Thank you both - I haven't successfully tested a fan-assisted work-out, fans not being essential household items for the British climate. iStat Nano widget tells me that the fan is running at 0rpm though I don't know if that is the norm for G4 iBooks when running normally. The only other piece of info is that the pointer can still be moved via the track-pad after the rest of the screen freezes. And at the last test there was no sign of a temperature spike on iStat. I'll see if I can extend work-time by some form of external cooling, and if that yields results, will go for spare parts option. Many thanks again. GSJ