Genie Garage door moving 6 inches and stopping

For a while now my Genie opener gets into a funk where it only moves a few inches and then stops.  Normally i just disconnect the door and open / close it by hand and then the next day it magically starts working again for another month or two.  Now it seems to be happening quite often.

With the door disconnected it acts the same way, moves a few inches and then stops.  So i don't think it is a motor overload or spring issue.

The limit switches for all the way up and down seem to be working OK.  There is no optical beam sensor for blockage detection.

I took the circuit board out and resoldered the header pin connector and replaced a tired looking electrolytic cap.  No change.

My best guess is something with the circuit that detects the motor is spinning is not functioning, but I do not understand the theory for how it works.  Can anyone look at my picture and tell what heck this thing is, how it works, and how to fix?  It looks like a magnet on a switch blade on motor wheel that spins which the CPU some how detects.  There is 15vdc on this blue wire which goes to the controller chip on the circuit board.  The metal motor wheel is grounded.

picture
https://i.postimg.cc/m2DgwCm2/20190212-145742.jpg