[mshac]

My 2 cents:

There is a much greater load on the flap motor in-flight than on the ground.

Could be:
1. Weak 10 amp circuit breaker
2. Weak motor drawing too many amps
3. Flaps out of alignment or needing lubrication
4. Sticky push-pull control cable
5. A short in the wiring somewhere, including the selector switch

Try to duplicate the issue on the ground by having someone place some resistance on the flaps when lowering. Nothing crazy, but enough force to somewhat simulate airflow.

If you are able to duplicate it, the problem is likely 1, 2, 3 or 4.

If you are not able to duplicate it on the ground, then I would suspect a loose wire that airflow is somehow causing a short, problem #5.

Do the flaps move at all beyond 10 degrees while in flight, or does the breaker pop exactly at the moment you select the next flap level?

You can put a meter on either side of the breaker and read the amps as you run the flap motor. You will have to remove the breaker cover to do so. If the amps are close to 10 while on the ground, they are for sure over 10 while in the air.