Try damping some new paper towel (not toilet paper) with denatured alcohol and sliding it in and around the gaps in the rings.
Lay the camera on a clean towel on a table and angle it so the paper towel is at the bottom when you run it around while you turn the rings. Any grit falls down on the paper towel, so use the towel only once around the ring.
This'll work providing you haven't had this problem for a while and you've ground the grit into the servo. If so it needs a trip into the shop.
Whenever you bring the camera indoors after any shoot, run a Dustbug over all surfaces then wipe the camera clean.
Especially do this around the top of the tape transport so when you open it to change tapes, dust and microgrit doesn't fall into the tape well.
This is biggest cause of tape dropouts. Hope this helps.
