The Trane XE A/C unit in the attic for the second floor stops running after apparently random times. Turning off the manual power switch to the unit for about 15 seconds enables the unit to start back up again. It appears to happen once it starts warming up, but then again it has run a couple of days straight during hot days (90 degrees +).

We had a lightining strike on a tree near our home in February, and we had controller boards replaced in both of our A/C units. However, the unit has performed very well until now.

A technician came out and thought it was a flaky door switch on the air exchanger. He bypassed the switch and it ran properly for about 2 days. Now it cuts off after running for a couple of hours. The compressor outside doesn't run when this happens, and the circuit breaker isn't tripped. He also checked fuses and he said that they appeared to be fine.

Any ideas of what could be happening? Or things to look at?

Call the service person back. They did not fix the unit correctly the first time and they should not charge a service call again.

If the control board is new and he jumped the door safety switch it should run unless.

The 24 volt transformer is cutting out.
A low voltage wiring problem. (make sure all connections are good and tight.
It is possible the circuit board is defective (just because it was replaced does not mean it has not gone bad again).
Something in the 24 volt control circuit is opening causing the unit to stop running. (the control circuit has many different components that should be checked,).

If your drain pan has a overflow switch that could be the problem. If you have one and most attic units do I would jump the two wires together temporarily to see if that solves the problem. (NOTE: if the pan is full of water the drain line is plugged up).

Sorry but without more info this is all I have,