isn't that alot of prop for that poor lil motor to be pulling? maybe overheated the engine. dunno i haven't run an Irvine but the ones at our club run the same sized props as our 40's
on secone thought i guess the 11x5 would be ok but the 12x4 might be tryin to move too much air

also did you change fuel types? nitro content? oil type? elevation? tank position? are you getting proper tank pressure from the muffler?
well thats all i can think of right now


john