My KX250 is doing exactly the same, started just after break in.I don't know how many hours on the bike, had it for about 2 months, ride every saturday and sunday morning from 9:00 to about 12:00.
If you need to change the rings everytime this is happening, you'r gonna spent more time doing maint and less time riding.Although there is a little oil dripping after every ride, there is no difference in performance, I still want to install a JD jetting kit and flywheel just to try and tame the beast a little.Also never repacked silencer, no excessive noise or lack in performance.
I am just going to keep on riding until lack of performance, excessive oil and smoke from exhaust or strange noises kick in, wich ever comes first.
By the way I am not racing just riding for fun.