FWIW, doing the binding check for the gearbox with the cover off proves nothing. I've seen a few of these gearboxes where the cover need a shim to avoid dragging on the worm.
I have great difficulty with three boards all failing in the same manner, that seems somewhat unlikely. Also, if it runs smoothly at higher speeds, that screams out mechanical issues, at least to me. I sure wouldn't suspect the board first in this case.
If your bandaid with the tach tape changed the running, have you considered swapping out the tach sensor board? Also, check closely the gap, it should be between about .8mm and 1mm. More or less gap will give you problems, and it doesn't take much. Don't eyeball it, measure it.
