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.

I don't think this diode is the culprit, TBH. Just out of curiosity I
hooked it up and tested it this afternoon. The faster diodes turned up so
I thought it might be instructive to compare them. The main flaw in my
test is that I'm unable to replicate actual working conditions. I just
hooked up each diode in series with a 1k resistor and fed the arrangement
from my 600ohm sig gen using 10VAC p-p. Slow recovery was certainly
visible on the scope with the BY134, but it wasn't *that* bad. In fact it
was still able to function as a viable rectifier right up to nearly
600kHz. There were no signs of slow recovery with the UF4007 of course,
but the difference at 20kHz, whilst still noticeable, is unlikely to be
causing the issues I've experienced.
But as I say, it was in no way a scientific test and only when the new
diode is in circuit will we know for sure. I won't be holding my breath!