TV cuts off w/o warning & power red light blinks continuously. Can cut TV back on but will cut off again. Sometimes it does this every few minutes and sometimes, especially in the evening, it will stay on awhile. Had a repairman come in but he couldn't determine why this was happening. Picture is perfect. What's going on?

This may seem like silly answers but...
1. Be sure the power supply is plugged in all the way and that it is not damaged anywhere on the cord all the way up to the TV.
2. Be sure the Sleep Timer is not activated. You wouldn't believe how many times this is actually the case.
If these end up not the case then it is very possible that the safety circuitry in your TV's board could be going out. The TV is shutting it down due to it finding a problem with the circuitry in the TV. Most of the time people choose to replace the TV with a newer model as it is just cheaper to replace it than to have it repaired. This usually only happens in older TV's although I have seen it in TV's as little as 4 years old.