I'm certainly no expert but I'd be suspicious of:
1. Collimation
2. Edge support
3. Something intruding into the light path... I've seen misshapen stars and then realised the dome was partially obscuring the OTA... derp
I've recently been looking into some optics issues and, from what I've read, a cooling mirror can cause over correction...
here is a good video which shows a star test on a known badly corrected mirror. Doesn't sound like your problem though.
Good luck Colin... optical issues drive me nuts. Hopefully yours is just collimation.