T61p power off during restart

[mitasol]

I have a T61p still in warranty that is having a strange problem. When I select restart or windows update does, sometimes the machine power off. It will not restart until I unplug the AC adapter and remove and reinstall the battery. Today I had the issue again and had a heck of a time getting it going again. The system would start, go through Windows startup and then power off, other times it would go to the last windows startup failed and then after selecting normal startup it would power off. Starting process of elimination I tried starting with no battery installed, only battery installed and no ac adapter and then with a different drive installed - same issue. Then I put everything back together again and it started - backing up as I type
I have recently had the system board replaced under warranty, the problem was the machine just wouldn't start - I suppose this means that I am going to have to send it off again, but can anyone think of anything that I may have missed?