I don't know what engine you are talking about, the engine I have and was talking about is a 4 stroke YS 120 SC.

Some more info would be a big help here to maybe help you deal with whats going on with your engine. This much I can tell for sure one of these items or more is your problem. Its to lean, wrong plug, fuel or over propped or even a air leak for it to be running hot.
For a engine to drop RPM's when running to hot is normal we call it sag and if you allow this to happen very often you will ruin the P&L in the engine.

Engine type and brand name
fuel and plug
Prop size and type