[Fischer_76_911S]

Phew, what a long thread. My two cents from far away.

1. Lots of good advice on here.
2. Static is a function of both return and supply. Door off reduces static, but that doesn’t mean problem was with return.
3. Advice given earlier to check evaporator coil and clean if needed. Also check if internal insulation around coil or furnace is loose. I had one Job where the insulation blew onto the coil from the side of the cabinet when running, threw a code, and then returned to its normal position at rest.
4. Check insulation around blower motor for same problem. Door on could equal insulation sucking in and blocking 50% airflow.
5. Good advice for locking down in low speed, but see above.
6. Replace motor if all else fails, especially if it is original. I’ve seen over speed situations on ECM motors, though very rare.

My $0.02.

Bob