I had a problem simular to this with my trainer. Engine ran good no problems with it at all. Had plenty of power for a 40LA. Fuel lines had no holes and even replaced them but for some reason the engine would quit while flying. You could deadstick it in and restart the engine and put it back up and fly around for a few minutes and the same thing would happen. What I figured out was the problem was the trottle wasn't set to the right length and then when I throttled back to far it would kill the engine. When I did my ground check everything looked ok. But like I said it was only when throttling back to a low idle did the engine die. Reset the linkage to the right setting and everything is great now. Also you might think if you've been flying in the cold the expansion and contraction of materials at different temps will cause the control rods to change lengths and cause problems

HTH
Brandon