are you sure it's the heat and not an electricity problem? my system was running without enough juice for about a month before the PSU finally started to give out and my system (or parts of it) would shut down... this usually only happens when you add too much hardware...
I wouldn't take touching the heatsink as any valid measure of temperature, because even a CPU running at 30°C is 86°F pretty warm to the touch... and like ober's, my CPU (P4) usually runs at somewhere near 140°F (60°C)