On Mon, 22 Feb 2016 10:57:06 -0000 (UTC), Cursitor Doom
wrote:

I should perhaps have been more specific and stated that V1811 on the
schematic is the diode that was incorrectly replaced by that lower grade
part.
Anyway, replacements now on order; will report back in a few days.

A gross failure in this part would blow a fuse, hence it is the least
suspect in that regard only - the fuse doesn't open.

It is actually the only one that is involved in power transfer -
seeing double input voltage stress and peak/average conversion
currents; the other two are snubbers/clamps.

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

RL