On Mon, 22 Feb 2016 08:49:11 -0500, legg wrote:

If it's slow, it looks like a short when the power transistor is trying
to turn on, stressing the current snubber around L1804.

Well it seems it *is* far too slow if I understand Dimitrij correctly.
Anyway, I've managed to source one of his suggested substitutes, the
UF4007 type quite cheaply online so we'll find out before the end of this
week if the wrong replacement part has been responsible for the problems
I've experienced.