We solved this once with a new guy at the field and it turned out to be really simple, Mike. His MDS would stall on take-off every time. With it running at full speed on the bench I noticed the muffler pressure line beginning to clutter up with oil. We yanked the line and the engine instantly picked up speed and began to run a lot better under all circumstances. Another point of interest is that while running at full speed w/o the pressure line connected, gobs of oil squirted out of the muffler pressure fitting. I haven't checked to see if this happens with other engines that do not suffer from "mystery stall" and I can only assume that oil stopping up the vent line is what causes the running problems, but everything works fine now that the tank is vented to the atmosphere.