iMac shutting off shortly after boot
So I have been working on this iMac as a bit of a hobby at this point.
Long story short...
Bought a 27" iMac on Craigslist, wasn't working, so I bought some parts for it (Power Supply from iFixIt, random screws and brackets, etc. and eventually a replacement logic board). Nothing worked until I had the new (well, older) logic board in and it worked great...for about a day.
Then, randomly, I check on the computer and it's turned off. Odd...I figured it had a sleep issue, so I power up and turn off the sleep timer. That ought to fix it. Nope. Ten minutes later it's down again.
Now it won't boot up unless I give it several minutes and then it will hit the chime and then shut down again. If i give it a few hours, it will boot fully and I can putz around for a minute before it shuts down again.
I'm thinking it's heat related. I ordered new thermal paste as the stuff I used on the i5 I bought was purchased originally 2 years ago (when I replaced the CPU's in my Mac Pro 1.1 to Quad Cores). But I assume heat because:
1. It worked great for the first day...no issues whatsoever. Installed everything, etc.
2. It works longer when it's unplugged longer.
I guess it COULD be the logic board, but it seems unlikely since it did work really well for awhile there.
Anyone have any thoughts on this?