PowerMac G5 fan problem?

Recently purchased a dual 1.8ghz powermac g5 for pretty cheap, and overall i'm impressed with the deal i got. it came with 2 gb of ram, but i upgraded to 4 gb.
the only thing is though is that when i first power it on, it runs fine for the first 5-10 minutes then the fans start to speed up until it sounds like a jet engine. i shut it down and turn it back on and it doesnt do it on the second start up...

anyone know what causes this problem?