I had the same problems and this was the fix. Sounds like your piston and rings are worn. (rings are not sealing properly letting oil into the chamber. Rebuild the top end bone dry - Dont cover any parts with oil, and kick the bike over 60 times with no plug fitted. Your rings should now be bedded and sealing well. Try and stick to the one brand of gas and depending on local rules run a good octane booster if your only running pump gas.
The first and the cheapest mod I made to my 06 450 was a 14 tooth front sprocket. For most mere mortals, 13-48 gearing is just too agressive. Going to 14-48 will tame the beast ever so slightly giving better hook up out of corners. I almost garantee this mod will make most riders smoother and therefore faster riders.