I never had much trouble with my GS15 engine. Maybe you should set the mixture needle to the factory break-in setting. Check your fuel line is not pinched or has a cut or pinhole. Check your muffler presure line for the same.

Set your radio throttle trim to "0" or neutral, and then looking at your trucks radio and linkage, double check that the throttle and trans linkage are not trying to engage a gear when the radio transmitter is in the neutral postion. Otherwise that can be putting an excessive load on the engine when it's trying to idle.

Also, during break-in glow plugs will foul easily and cause all sorts of running/idling problems. Be sure your fuel is fresh, and you have a 100% known good glow plug.

Don't give up on the Mega Force! It is an awesome truck and when you get it running reliably you are really going to enjoy it!