hard to say for sure, but if refilling the tank solves the problem then its the "mid tank lean"

I cant say I know for sure what causes that, but Ive seen it with models Ive had before. A good tune at the start becomes lean as the fuel in the tank hits 1/2 to 1/4.

With any nitro issue, I always look for leaks to start with. A bad tank lid gasket, carb seal, exhaust gasket or any other gasket for that matter can affect the motor's ability to suck in fuel.

Truely, a small thing like a poor gasket on the tank cap can cause issues.

If the motor can be refired after refueling...and the power is always good the 1st 1/2 tank, then Id think the motor is sound, and the issue is elsewhere.

How does it feel to pull the motor over? If compression is low and causing the issue, you can feel it just by pulling the motor over.