Ben; I've the same problem with a GMS 1.20 and solved it by changing the propeller. I found that it's a thrmodynamic problem... but to summarize: you're not absobing enough torque so the engine gets really hot. Once this hot temperature is reached fresh fuel into the crank looses its properties and combustions cannot continue.
Try a bigger propeller; you've nothisng to loose with this test...
Good Luck
Guille