your engine is uncowled, the pipes on the outside, you have two control horns at the back, thats drag. but i agree the wing is now producing too much vortice if its flying at a certain AoA to get lift.
try sending a PM to.. erm... johnd and ask him where his is trimmed.
last thought, the wing has a slight gap on the extention at the rear, if you've closed this by re-profiling the fuz, the wing will be at the wrong angle