[wilf leek]

I wonder if this is actually a fuel starvation problem? Is the fuel pump not keeping up with demand? Blocked filter in the line somehwere?
Have you ever kept it WOT in top at high revs for a reasonable length of time? that would be another time when lack of fuel flow would show up.

It's the "Ok after a couple of seconds" thing that is leading me down this road. Could be worth fitting a fuel pressure gauge and seeing what that shows during the "fault" condition.