psuryan

I have found that for large craft, the issue is having to many engine gimbles. It just starts the craft swaying back and forth or rolling. For me, disabling all but 1 or 2 mainsail gimbles usually fixes the problem. It also sound like your rocket may need a few extra SAS modules. It may not be turning over fast enough. It has to turn to prograde in 30 seconds if you let mechjeb time warp. The other things is that mechjeb will point it a bit nose down when circularizing the orbit. The pulls the Pe up while keeping the Ap constant. Otherwise, as you pass your Ap point by more than a few seconds, a long burn will also raise the Ap. This down angle keeps moving, making mechjeb do more work, adding more error at times. you can use mechjeb to just keep the craft pointed prograde, and do the burn yourself. you may end up with a small error at the end, say a few km above your desired Ap. Just use the circularize option to fix it this rough orbit, this short burn should not have enough time to cause any errors. And one last fix. If is starts to wobble way off course, disable mechjeb autopilot, hit the prograde button until you are lined back up, and reengage autopilot. 50/50 shot this will work, I have found sometime mechjeb just needs to get itself reoriented after starting the gravity turn. But yeah, preferably, add more SAS to your lower stages, so you lose the weight when you drop the tanks. Then on launch, disable the gimble on all but 1 or 2 mainsails.