This unit shuts itself down after a minute or so. If I try to start it back immediately it only runs a few seconds.

I noticed the Heatsink holder (plastic) was broken and not allowing any cooling to the CPU and replaced it.

The unit and fans are all clean.

I changed out the Memory sticks with no change.

I replaced the ac cord with no change.

Any suggestions?

did you put thermal paste between the heatsink and the CPU.

Nope; I didnt have any. I thought about it but hoped it wouldnt make that much difference. I assumed that the heatsink being attached right again might work.

Guess not huh; maybe.

na you should apply some themal paste it makes a difference. unless there is something else wrong with the computer, but the first step would be the termal paste.