I think the loosing rpm thing is overheating. As it heats up to it's maximum opperating tempurature, the piston has a tendancy to expand faster than the sleeve, which is why it will freeze up. Because the la series is not ringed, i think the drop in rpm is due to the tightening fit between the piston and the sleeve, which adds even more heat to an overheating engine. If the engine is cowled, you may want to consider increasing the cooling effect and see what happens. Also, if you are using an aftermarket muffler, it may be slightly too restrictive for that particular engine. Two strokes need enough backpressure to equelize to a certain degree the pressure behind the piston with the pressure on top of it. If there is too much, It will have a hard time expelling hot gasses, causing it to overheat in muinets and die. It can't hurt to check it out. Dust inside the cylendar would have trashed it by now, and it would either run like crap or not run at all. If there was dust inside the cylendar, It would have been ground between the piston and the sleeve, scarring the metal, increasing blow by, and greatly decreasing the engine's life. Unless you put dust in there, there is no way it could have gotten in there.