Couple of questions Pete. What's the image scale on your guider set-up. I mean what are we looking at? +/- what? Regardless DEC has to be as tight as you can get it without binding. Too much backlash in DEC is not advisable. Your graph indicates a very rapid shift in DEC with PHD bumping the mount real hard on a periodic basis.
That can be a tight spot in the worm or a balancing issue or a bad alignment causing a shift too rapid in DEC, or as simple as a greasy clutch disc, but the drift would be random and continuous.
To diagnose these things the first thing you need to do is disable the guide output in PHD, do a basic drift alignment and see what happens without guiding then take it from there. There are too many variables to try to diagnose everything at once and you need to go through a process of elimination.
Forget what you did on the night thinking I've already checked that. You probably were tired, frustrated, wanted to start imaging asap and missed a lot of things. Been there done that.
