One thing I might try, when the problem seems to crop up, would be to slew to a distant target with quite a change in RA and DEC as well. This would give the axles a chance to slip quite a bit, if they are indeed the problem.
Then slew right back to the starting target, (but without a RCAL), and see if it is still dead on, or has now acquired a shift, which likely had grown even larger on the return slew. I think this would indicate an actual axle slip, and if you make note of the direction that each such back & forth slip increases, you will then know which clutch needs tightening, or if indeed whether the main counterweights need to be jogged a bit, or the OTA saddle position is significantly imbalanced. The other possibility might be a very slight “mount wobble” on the pier, caused by a general imbalance. That might be indicated if the back & forth slews to the start and end targets, doesn’t change – just a mechanical repetitive “leaning” side to side, instead of a slip of clutches, or OTA in its rings – or indeed even the camera in its focuser’s drawtube.
If it was an OTA imbalance, do you recall whether you actually balanced it AFTER attaching the camera, before leaving the remote site, or just trusted that it would not cause a significant imbalance? The AP mounts are nicely “over-designed” and can handle significant imbalance, even overloads, but the tension of the clutches could change just a bit through temperature cycles over the seasons, and thus weaken their initial grip. That might pose a problem if you don’t rebalance, or at least “snug them up” on each visit to the remote. You can’t simply assume everything “seems” to be tight enough, as it had been.
The old (white nylon) clutch plugs should never be pressed in too hard, since the plastic material does “creep”, flattening itself in the hole, and eventually compresses right through the hole, sitting partially astride in the narrow space between the outer axle hub body and the inner shaft itself. thereby “shoulder-wedges” itself like a mushroom cap, in the clutch hole, making it nearly very difficult to loosen the clutch ever after, and nearly impossible to extract the plug with ANY tool to replace it. – Based on personal experience.
So, it is a fine point, whether our tightening down of a clutch is too strong or too light – eventually, they compress, or change shape, sometimes hairline fracture under constant pressure and with temperature variations, as well as just with aging. I suspect the new series AP mounts, without clutch “nylon plugs”, perform better under such conditions.