Well, I used two different fully charged batteries in both the plane and Tx with identical results. Interference is unlikely as I am very rural (and other plane with different Tx/Rx is unaffected), and the Rx wouldn't respond even from three feet away after it acted up. However response was fine for the first few minutes into the flights.

There is no cap on the motor, I was just asking whether there should be. Turns out they sould only be installed on brushed.

I've sent the Tx back to Hitec per their suggestion. Hopefull they'll find the root cause.