iMac G5 keeps freezing

So, i found a 20 inch ABL iMac G5, but there was no PSU. So, i took the psu out of my 17 inch model, and it was giving me a hard time booting up, but when it did, it worked for about 5 min and then it froze and the fans went insane. Any ideas? I already tried resetting the pmu and smu or whatever they are called.