my RM250 did that a few weeks ago, and gouged my cylinder up pretty bad(in my opinion anyway...), I sent mine down to Eric Gorr who overbored it, cleaned it up, and redid everything...
to be very honest, with 75% of my riding being trail riding, and alot of that riding being with kids, I was kind of leary at the time about overboring that cylinder like that(for reliability reasons) and would probably have just prefered to stay stock with it, but after talking to him and getting it back and riding it, I'm glad I had it done...pretty awesome job, and for that bike it was alot cheaper than replacing the cylinder...
I have friends who have had work done by him, but I personally have only talked to Eric a couple times on the phone and this is the first time he has ever done any work for me, and I have to say his work is first rate, I was very impressed, he had everything all set to just bolt up and ride, he even had the power valve adjusted spot on, I didnt have to mess with it at all...
if you havent already, I'd give him a call and see what he says... I know he's got my business from now on...