Hi!
The sympthom you describe is related to that the engine doesn't get enough fuel and is overheated. Simple as that! Check if the engine can hold its high speed setting when you hold the airplane straight up....
Check loose screws when engines is cool,not hot or warm! also check bad fuellines for cracks and that all lines are as short as possible. Never use an in line fuel filter on board the airplane ...always filter the fuel when fueling.(Use a big paper filter inside the fuel-jar in your field box).
You should also put in a new Enya 3 or OS 8 glowplug. Prop 10x6, or preferably 11x5 or 11x6 if you fly a high winged trainer, APC props are recommended.
Fueltank 8 oz, mounted so that the center of the fuel tank is inline with the carb when plane sits horizontal.Very important! See to that you use lots of soft foam rubber around the tank. Never mount a tank direcly to the fuselage formers like seen in so many ARF airplanes.
Always use 3 line fuel system with two clunks running parallel, one to the carb and one to the pressure nipple on the silencer. Third line going to the top of the tank is for owerflow.
This tank system is called a "Uniflow" tank set-up and it is better than an ordinary 2 line tank hook up.
Never use a plastic spinner as thse are notorious for giving balance problems.
Also check fuel.
Hope this helps.

Regards!
Jan K
Sweden