Mine had the issue while out duning, 3rd gear variable throttle.  Find a big bowl, start coming down the dune approaching the bowl, shift into 4th mid throttle, hit the bottom of the bowl, and hit W.O.  Let the engine / speed spin up and shift into 5th, again W.O. and after a few more seconds it would fall on its face, so I would let off the throttle, start coasting for a few seconds and then could get into half throttle again in 5th or downshift into 4th about half throttle again.

I could not experience it when drag racing up Olds or China Wall as those pulls were too short.

Let me see if I can dig up a helmet cam video when this happened.