"I have an Intel 24" Imac (late 2008) that was giving me lots of problems although it worked fine if I used a second external monitor. "
But at that point you didn't reveal if you had that issue fixed. I'm guessing you did not and now want to start a career in PC repair. Since the power support can be swapped out and may be the cheap part you are following some one's sage advice. Try the cheap parts first.
I have an Intel 24" Imac (late 2008) that was giving me lots of problems although it worked fine if I used a second external monitor. Then one day it just stopped - it would restart only to go black and shut off. All searches indicated it was the power supply. Installed a new power supply and it still will not start completely. I did and extended hardware test and it failed to complete the extended test failing at the end with this error message: Apple Alert 4HDD/H/40000004=SATA(0,0) I tried booting in safe mode, nothing. I tried booting to Max OS and I get to the place where I choose HD or CD and I choose CD and after a short while nothing...white screen. If I just start the Imac normally it gets past the little gear only to just shut off and the screen black. Presently I have a white screen and the completer is locked up. Before all this the HD did work. Changed out the monitor last time also. Not much else---Any suggestions