What type of distributor? Should be about the point in acceleration where the vacuum advance is approaching it's max, and maybe the mechanical advance isn't quite coming on soon enough... Initial timing and timing with and without vacuum, as well as the mechanical advance rate might be your stumble. The total timing might be a bit high, but if it's not rattling or falling over with 40 degrees cranked in then it should be ok, too. Have you tried the vacuum advance hose on a fitting above, then a fitting below the throttle blades????