It's that strobe that I needed to use a fet to commutate the xenon flash. Actually now the HV transformer is being fed by a fet half-bridge. I didn't even power the drains to see the problem.
I saw the real problem- I can't believe I was this stupid! 0.1uF ceramic is not enough to stabilize any reg. I have a MIC2950 which is described as a nifty"bulletproof" reg with a low output capacitance requirement, but it requires "only" 1.5uF. It was bugging me as I went to sleep last night how driving a gate capacitance could produce >5v on the rail. I realized it was impossible and the reg stability had to be the problem. I don't know when I decided a reg could work on one of the cheap ceramics I have so many of, I need to check my other designs to make sure I didn't do this.
MIC2950- great micropower reg, handles voltage spikes over 60v, low quiescent current, low capacitance requirements. Most often comes in a TO-92 pkg. **broken link removed** carries them cheap.