More considerations...if all else checks out OK.......
Engines that are overly sensitive to the usual changes in head pressure as a tank empties in flight aren't pumping efficiently. There just doesn't seem to be a needle setting that will keep them running for the entire tank.
In other words, crankcase pressure can be leaking out the front / back / top of the case or the piston isn't pumping up enough air to begin with.

Changing the plug is of course the easiest thing to do first. Sometimes the old copper head shim is leaking.

In general it is good to have a gun cleaning kit with a plastic bristle bore brush and Hoppes solvent to keep these Cox engines devarnished.
Trouble is, a little varnish might be a beneficial "bandaid" on a worn engine.!
You can use a cheap magnifying glass and a flashlight to inspect the P/L surfaces to see some pretty revealing flaws sometimes. You can also pressure test 2 cycle engines under water while looking for air bubbles leaking out of nooks and crannies. I've never tried this with a Cox engine, but with clay, duct tape, a drinking straw, you can give it a try.
The replacement P/L set I got recently from Cox International was a good one and reasonably priced if you determine it's time for a new set..