Shuts down

All the other micro hoods that I have owned over the years turn the exhaust fan on when they get hot from cooking on the stove.

The problem that I am having is this unit turns off as if it were unplugged from the wall. Is this normal or do I need to replace a part?

The unit was repaired under warranty when it would not heat. The replaced parts were: magnetron, capacitor, and control board.

Your help in resolving this issue would be greatly appreciated!
Thanks,
geocar