Hmm.. Could be a lot of things.. As a very general tip, a bad clutch or bad clutch bearings can screw up how an engine runs - bad idle etc..
If there was mud and gunk and stuff in the engine, and you ran it like that, that could be bad.................
You could also try a new plug. If you've taken everything apart and put it back together, it could be that something somewhere isn't completely sealed..
A new air filter is needed too of course..
Could be OK - probably not if you ran it with mud inside it.. Lot of stuff to clean, check, etc..
An LRP Spec 3 .2/Losi 454/Dynamite Big Red would be good budget engines to look at... There are of course tons of options for a wide budget range too though..