ORIGINAL: Charlie P.
Sounds like excessive wing incedence or way too little downthrust. If it loops with added throttle there is little else it could be.
Currently it has 5° downthrust and a wing incedence of +3°. Does this seem about right? Both numbers match the planes.
And just to clairify it will not loop with just additional throttle, if anything I would call it extreamly pitch sensitive.