This evening, while videoing Saturn, I noticed the image irregularly jerking about 5" in R.A., as often as two or three times per second, and then noticed I could hear a faint tapping noise from the mount whenever a jerk occurred. (This might be quite audible in a place without so many crickets and tree frogs.) The power consumption, normally 0.8 amp, was 1.1 to 1.2 amps, apparently fluctuating faster than my digital meter could keep up with.
A few minutes earlier, I had videoed Jupiter with excellent tracking.
I looked for a dragging cable, but there was none. I looked for a balance problem, but it was in balance (a tiny bit east-heavy) and slightly changing the balance didn't alleviate the problem, though it did make the taps a bit less frequent.
I slewed to several other areas of the sky and the problem disappeared; there were no taps, and power consumption was 0.7 to 0.8 amp. (I didn't evaluate tracking.)
What's going on? Until this, the CGEM had served me very well, and I was thinking of having it hypertuned simply because I'm the 3rd owner and it has never had any maintenance. A few years ago I had a tapping problem with my AVX that ended up being a bad gearbox. If this is going to be a multi-hundred-dollar repair I'll have to seriously consider getting a different mount. (And the advocates of CEM40 and GM811G can fight it out, giving me advice.) Or is it a freak problem that will never come back?