[PJL]

It's the blind leading the blind...

The kind of fault I think we are looking for is something like a valve going into grid current or one of your new diodes being too slow. I guess a decoupling problem is another possibility. The tricky bit is how to find it without a scope?

Disconnect MR1 and try again.

PS: Another interesting feature is the overshoot is occuring on both black to white and white to black.