I had a Magnum 52 four stroke that did exactly as you describe. Iended up switching over to an O.S52 four stroke and the issue went away. Mine would drop to around 3500 rpm and then slowly settle down to around 2300. Very slowly. Hobby People tech suppport replaced the carb and It solved the problem for a day or so. Then the old behavior came back. My guess is that their is an air lean in the carb, either High speed needle, mix control needle, or the carb to intake manifold. I never got the problem resolved and I now have an O.S52 carb on the Magnum and all is well.

The recomendation to Fly it like it is, is OK as long as the engine will idle down by approach end of the runway and hold a good idle and be able to respond to an emergency application fo full power. If it won't do this, fix it before you fly. Mine wouldn't,

Don