On what kind of airplane do you have your Tower Hobbies .46 engine? It sounds like you may be getting some fuel foaming. This is when the fuel in the tank is agitated by the engine's vibration, which leans the mixture. With some aiframes, the leaned mixture smooths out the vibrations, the air goes out of the fuel and the mixture richens. The engine slows down and the vibrations start again. You get a cycle of RPM changes.

Sometimes the engine goes lean enough that it overheats and quits.

You may also have the needles of the carb not in balance. Start out with a very rich idle mixture. Get the engine running and set the high-speed needle at full throttle. Then retard the engine and get the idle mixture set. Open and close the throttle several times. You would adjust the acceleration with the idle mixture. If you have to lean the idle to cure a rich acceleration, you may want to increase the idle RPM a hair to compensate.

Many times, people get a very, very low idle, but have handling problems. A bit higher idle can relieve a lot of difficulties.

If you still have the problems with the engine on the model, try it on a good, solid test fixture. If you still have problems, the engine h as a problem. If they go away, it's something in the airframe/engine/fuel system combination.