GS-1 shutter cock slipping

When using the speed grip with my GS-1 I often find that something is slipping when I advance the film. Under normal conditions film advance and shutter cocking require two cranks of the advance lever. When mine begins to slip the count number in the frame counter advances normally and I can see that the base where the hand crank normally connects is rotating properly (I can see it because I don't have the cover that you're supposed to put on it when using the speed grip). But after two cranks of the lever it doesn't lock. The frame counter stops at the proper number but the lever doesn't lock and I can keep cranking it. So I'm guessing that it is failing to cock the shutter. The solution is to crank it very slowly and gently and it usually succeeds in cocking the shutter and locking the lever after one or two tries. Frame spacing is normal and consistent.
Does anyone know why this happens and if there is an easy fix. I'm not too worried since I can still get it to work alright but if it gets worse it may be a problem.

It happens with any or all of my backs and lenses so I'm pretty sure the problem is in the body (or possibly the speed grip).

Probably fastest to just try a new grip. That would at least localize the problem. Grips are pretty cheap.

I think what I'll do is try shooting a roll with the hand crank instead of the grip. But I'm pretty sure the grip is okay... I'm afraid something is slipping in the body where the action of winding turns the gears that cock the shutter on the lens.