Synscan (in EQ mode) won't track in Dec anyway, you have to have good polar alignment or the stars will wander off in Dec. The fact that your tracking error is in RA only seems to prove that your polar alignment is good.
Will the displacement in RA get continuously larger over time, or does is oscillate back and forth? If it only goes one way (too slow) than a gear slipping on its shaft is a good lead to follow, especially if the RA axis is stiff. The gear on the worm shaft can be tightened with two set screws, but unfortunately the sprocket wheel, once loose, cannot be tightened on the motor shaft. Sounds like your mount could be a good candidate for a belt mod
Cheers
Steffen.