Power management issue

I just got back from techserve with my Black Macbook with a bill of 0 dollars as they could find absolutely nothing wrong with my power management system. 3 days ago my macbook suddenly turned off - went black and repeatedly tried to turn itself on over and over again until I unplugged it and removed the battery. I tried to drain it by tuning it on without the battery but, after plugging it in again and trying to start up again, could not get past the grey screen without it turning off again. So, I let it sit there for 2 days until I had time to take it to be repaired. The rep pressed the on button and the computer turned on without any problems. In fact, other than a few directory problems, they could find absolutely nothing wrong. Thus, the free repair. So, all is well except for an about an hour ago, the computer turned itself off again and acted like it was doing a PRAM reset on its own and then turned back on. Any ideas as to what might be going on?