Be sure to double check all hardware. Maybe the EA pin is left floating. I spent the entire last weekend figuring this out when my programs would work for 1, 2, 5, even 30 minutes before starting to behave strangely. Sometimes it would reset, other times it would go to sleep (oscillator stopped). I found out the hard way, wasted alot of time, and ended up feeling stupid, all because I accepted hardware from someone and didn't double check it first. Good Luck.

Chris