A&A Pop Up Problem (tuffy)

[Yeller]

I have 1 pop up when the head is in, the gear set stops at that location. When I remove the head the gear set moves and all is ok. I have replaced the gear set - no change. New heads still stops. This is an A&A top flow 5 port with balls. I have removed other heads and blew out scale chunks..not sure whats left. I have moved the balls around and still stops at same spot..There is no valve or flow restricter on the top flow port. All powered by just pool filter motor..worked fine before for 4 years..

Any ideas?? Thanks