I’m not an expert, but ... a few thought.
Have you routed your cables “through the AP-1100” and have you eliminated the possibility of “cable snag”? Maybe give them a bit of slack.
Losmandy HD Tripod:
I noticed that you said your Losmandy legs are locked down, after any adjustment of their extension pipes. Did you check the tightness on both the upper locking knobs (i.e. tripod head attachment), as well as the lower set at the feet?
I also use a standard Losmandy HD tripod for an RC-14 truss on my AP-1200, (OTA plus saddle etc., 74 lbs., similar to your C14 at 60 lbs.).
But to make sure it would not slip, ever so slightly, I cut three identical length wooden dowels (chop sticks, but can also use pencils, thick nails, etc. ), and inserted them right into those “leg slide” channels in the pipes, near the ground, to act as “stops” so that the leg extension won’t shift under all that weight - then locked the knobs there. This gave me an identical leg “extension” that can’t slip, even if those knobs loosened their grip. This could be a cause if the scope is set up quickly in the field each time.
Since my Losmandy has leg extension lock knobs rather than lever handles, I used a “rubber strap wrench”, for leverage to lock the knobs even harder than just by hand.
Make sure that both the upper and lower leg lock knobs are very tight, since if they have become slightly loose, the Losmandy HD tripod head can shift a bit or wobble, depending on where the OTA slew ends.
Could this be a camera attachment issue, with the CCD rotating, or sagging slightly as the OTA angle changes during a slew? Give it a gentle hand twist to see if it is indeed firmly seated in the draw tube, if using an eyepiece adapter. If the CCD is threaded onto an adapter, make sure it isn’t “unscrewing itself” due to the weight of the camera. This is often a problem with a CCD whose centre of gravity is offset – like the overhang on an STL-11000, especially with an of axis guider or filter wheel.
Since the drift seems to happen right after even a short slew ... What planetarium software are you using? Could it be that it is correcting differently for year J2000 coordinates than the APCC?