As nearly everyone else has said, the 11X6 is too much prop for a normal 40 and the LA is a little weaker than that so it's definately too much prop so start with changing that to a 10 X 5-no more than 10 X 6 for sure! As for this being a "wierd" problem, there's nothing wierd about it really as this is exactly why you hold a plane vertical. You are looking to see if you have this problem. It is likely that you need to richen the high speed needle a few clicks even if it means giving up a couple hundred rpm when level so that when vertical it won't lean out so much and quit. THAT condition will ruin an engine in short order if you routinely fly it that lean.