Paul Lloyd:
Neil,
Check that the Meridian flip switch is turned ON in Autorun Shooting schedule screen (3rd line in LH pane). If it is, then you have a weird one! and I would raise a bug report with ZWO if I were you.
BTW: That guiding RMS is quite something! How do you manage it?
Paul.
Doh! That was it! Thanks, Paul! I had not seen the Meridian switch in the AutoRun settings. Your reply came just in time because I am due to flip in 10 minutes. I paused, set the switch, and restarted.
With regards to my guiding... I wish I could tell you that I did something really clever. But the reality is that I think I just got lucky with my mount and rig. I have a stock HEQ5 Pro mount with no tuning or modifications whatsoever. I have a pretty heavy load consisting of a StellarVue 102 Triplet, a (heavy) FeatherTouch focuser, Nikon Full Frame camera, ASIAIR, EAF, and 50mm guide scope with a ZWO ASI 120MM mini guide camera. All on a heavy Losmandy plate. The total weight of my rig is about 21 pounds which I think is borderline too much for astrophotography with the HEQ5 Pro.
The only trick I did steal from these forums is to set my balance very slightly "tail heavy" so that the gears are constantly engaged. My understanding is that this reduces backlash.
And actually I often get guiding in the 25" range such as right now:

Many thanks again for your timely response!
Regards
Neil