all magnum two strokes i have delt with (and a few four strokes) take a long time to breakin, the magnum .52 in peticular. the engines run fine above half throttle but between half throttle and idle the engines would die. The first magnum .52 i got so fustrated with, it wouldnt transition worth a darn and would always die on me so i just started running it at full throttle. after 20+ or so flights the engine now runs great, the transition is perfect and the idle is very low.

The key thing i figured out with magnum engines is first off patience, if it isnt running perfect just keep flying it where it will run (aka fly it at full throttle)
second the idle changes EVERY FLIGHT, literally, after flying it at full throttle for 10 or so min the engine is breaking in, less friction means less fuel. Dont try and tune the idle perfectly because you cant (trust me ive tried for hours), just run the engine and after 5 flights lean the idle alittle and repeat. after two gallons you will have a very good running engine

my magnum .52 four stroke was also like this. i have two magnums .46 that have the same problems but to a lesser degree.
and finally my friends magnum .52 had the same problems mine did, just needs alot of breakin time

hope this helps