Sadly, as you have already found out, TH engines leave "something to be desired".

As was suggested above, I would return the engine and get something else.

Although it may cost a few extra dollars, you will not go wrong with any of these engines:
OS FX/AX, Thunder Tiger Pro or the Evolution 46 engines. 99%+ of these engines run great right out of the box.

If it is any consolation, I made the same "mistake" several years ago when I got back into the hobby. I USED TO consider myself pretty good with engines (actually I am pretty good with them), but the TH 46 engine just kept having problems. I would correct one problem and another would pop up. I finally gave up on it and the engine is now used as a paper weight and a "reminder".

I would also suggest avoiding AT ALL COSTS, MDS and MegaTech engines. While I have not owned any of them, everything I have heard and read about them make the TH engines seem like a Cadillac compared to MDS/MegaTech, which seem like a trouble plagued Yugo.