Finally got Panther engines (experimental) and test flew my first real supersonic jet. Things were going good up until 550m/sec+. After that, small corrections would create an oscillation that the SAS had one heck of a time compensating for (Jeb had his hands full to say the least). I was able to smooth it out by geeeeeeeeeeeeently straightening the plane out (roll or pitch adjustments) but it was temporary before they came back.
This doesn't seem to be having an adverse effect, just makes it look like my plane is having a seizure. What helped minimize it was assigning pitch/roll/yaw to specific control surfaces only (ie: control surface A can only do pitch controls but no yaw or roll) but it didn't negate it. I know the Panther engines vector so I was wondering if all the various controls weren't working well together and needed to be tuned (ie: turn off gimbal control on the Panthers or at least minimize it). The plane design is extremely stable because I managed to get the Cg to stay in the same place when fuel is being consumed (it moves a small amount but towards the nose of the plane).