Balance/weight? I found my cgem dx was VERY fussy about balance and alignment accuracy. I also joined a beta celestron firmware group and firmware made a big difference too. My start setup was polar aligned and slewing to alignment stars from there was a problem like yours when I first got the scope, after beta firmware update it was good. With the 11" cas it was at its worst, if i swapped on a smaller lighter scope/load alignment would be reliable. Also make sure you're alignment stars are properly identified and your finderscope centers on whatever is centered in the eyepiece. I found balancing with the scope pointed at what i want to photograph helped. I'm sure the motor/power strain differs as the scope changes position and it doesn't take much to put things out.
Also some ideas on
http://www.nexstarsite.com/OddsNEnds/ASGTRunaway.htm. Try get your balance as good as possible, use the longer alignment options available, I would go with more than 3 calibration stars, small time cost to do at the start of an imaging session. Also try to get firmware updates, the official celestron site seems to only have "final" firmwares that are years out of date, but the beta fimwares are available elsewhere and are stable enough to get rid of many issues (I think i got onto them through a post on Cloudy nights form, it was not a celestron url it took me too, might have been the nexstar site but cant recall from memory exactly. But it made my mount so much better and accurate, initial alignment were noticably more accurate and track much better too. There were a ton of fixes for southern hemisphere users.