Pat,
The YS .91 I have is the first and only YS engine in my stable--however, I have 3 more on order-- a .63, another .91, and a 1.20. Have been so impressed with the present .91--the sound, the power, the throttle response, etc., that I thought I should try some of the other sizes. Anyway, I have about 2 hours running time on my .91 both on the bench and in the SpaceWalker and have had absolutely no problems with the engine. I own 8 Saito 4 strokes of various sizes and have had some symptoms similar to yours with them over the past year or two. The quitting in mid- run, if not due to fuel starvation (running out of gas), has almost always been due to overheating of the engine for some reason or leaning out due to me not setting the needles rich enough on the ground prior to flight. If I had held the plane vertical for a few moments before launch, I would have discovered this problem with the leaning out in the air. I fly by myself much of the time and it is hard to do this holding the plane up and operating the throttle routine with one person. Anyhow, I hope some YS expert will step forward and give us an answer as it may happen to me sometime. Did you change anything on your setup such as prop, needle adjustment, cowling change, etc. since it ran well?

Sam