I'd like to know too, as I am getting this happen also.
I'll do a VERY accurate polar alignment, and do the calibration. All looks fine, but it starts drifting off with PHD not even trying to do ANYTHING at all.
The other night, I got PHD to guide PERFECTLY for 3 hours on one target. My polar alignment was bang on. I moved to the next target only a few degrees away, set in the menu to Force Calibrate, and started again. PHD did a ridiculously short calibration before turning green, and then just wouldn't guide. Stopped it, went into the live viewing mode without guiding, and watched the star. NIL movement over 5 minutes (and I did definitely have it in live mode
. Hit Force Calibrate again, and same thing - PHD literally pushed the star out of frame!!!
Closed PHD, started from fresh. Same deal.
Closed PHD, ran subs UNGUIDED, and got DECENT almost trailess subs for 5 minutes.
Not sure what is going on in PHD, and I tried all the calibration step changes etc, made it aggressively track etc. No dice.
Since the latest update, PHD has been odd for me. May need to try Starlight Xpress' Lodestar software... but what a headache that program is compared to PHD! Maybe Maxim (OUCH!).
I am going to write to Craig to see if he can suggest anything. Perhaps I'd be better with a SHORTER FL guide scope?