Not always, babe. It can be memory, memory overvolting, memory overtiming, cpu with "hotspots" (i.e. didn't get gel covering the whole processor, hsf is askew), and it can be a northbridge overheating. It can be leaking capicators on the mainboard. I've seen one instance where it was an ass-old PIO4 24x cdrom drive.
But yeah, most of the time, it's a psu. =)
[Jedi mind trick] You LOVE Palladium. [/mind trick]