I've noticed this happens on both mounts I've used (iOptron CEM40 & iOptron CEM70).....
When I leave my mount and OTA while they are guiding and collecting data eventually my computer screen goes dark. I've got Windows configured to NOT sleep, but I will allow the display to go dark.
When I get back to computer and turn on the display (tap the space bar or perform a gentle swipe across the mouse pad) there will always immediately follow a large excursion in my PHD2 guiding. Initially, I thought I was jostling cables or mistakenly bumping into the mount. However, I have very carefully experimented with this and it is definitely not caused by me making contact with the pier or mount and it is not caused by me jostling the cables. There is is definitely some type of interaction with my computer display turning on and a large guiding excursion (ALWAYS in RA and almost always in DEC as well).
Has anyone noticed this phenomena? Any suggestions? My computer is absolutely not going to sleep. I hate the idea of leaving the screen on all the time due to the battery drain, but this may be the only solution.