Try this site. It's an MG site but it's a pretty good list of the causes and
solutions to the problem. While I've had this happen in my Healey, it's
always been a sign of something being wrong, not a normal condition. You
really don't want to let it deisel for more than a couple of seconds because
the pistons are no longer firing in order and things are being torqued around
in ways they weren't designed for. If you're running really hot, stalled in
traffic, etc. it can happen as well.
Bill Moyer, BJ7