I have a heq5 pro mount (with rowan belt mod) and have all but given up on astrophotography because of this guiding issue. Scope is a 100mm Apo. asi cameras (asi 533 and 120mm for guiding) using eqmod pc to mount (and also asi air plus).
Essentially I have these random spikes that show up on the guiding graph that result in a double image of stars a few star widths apart. They happen frequent enough that shooting at over 30 second exposures results in half the shots being ruined
PHD2 guidelog image attached. (note the final part of the image I changed dec aggressivenes to 5% hence why that correction takes so long)
So things of interest, spikes are on RA and Dec! mostly RA though
Spikes are both above and under the graph (so tracking can be ahead and behind). Between spikes guiding is quite good.
(errors on Dec and RA on either side is what really confuses me)
Errors are fairly consistent in size 12.5 arc seconds. (Note <for scale> if I tap the guide cam with my finger nail I get a 4 arc second spike.)
Its possible this started after a night of the telescope mount being stuck grinding the telescope against the mount.
Things I've mostly ruled out.
Power supply, tried 3.
USB cables, tried 2 (no hub in use)
Computer, tried PC and ASI Air plus
Wind/wires/wobbly random bits.
I've opened it up and cleaned/regreased
I've tried east heavy/west heavy
I've tried minimise backlash, I've tried loosening effectively adding backlash.
I tried switching the RA and dec motors
I can hand turn the pullies freely through full 360 on RA and dec without binding or overly heavy resistance
Slewing is fine
Balance is good.
Calibration is good.
I'm kinda thinking main pcb issue bit I dont know.. and its an expensive guess.