Rog,
My sons Magnum15 had the same problem from new, after buying 3 gaskets I lapped the exhaust and muffler faces back to flat and installed the muffler useing permatex No3 non hardening , also used it on the threads of the muffler bolts. As the permatex dries sticky it prevents the bolts vibrateing loose and also prevents them locking in the alloy.
Nice looking model , too many overlook the joys of smaller models, my Trainer was/is (I still have it) an Aeroflyte 15 Trainer 48"span 3ch R/E/T Enya15 TV R/C powered 8x5 prop nice combo.
Also Have an Old COSMO 10 ,42" span 4ch trainer AP09 another nice combo- sad they are no longer kitted.
Stewart