My wife's '01 YZ250F has started running like crap, mainly at higher elevations. I know engines in and out but am fairly ignorant when it comes to carbs. Everything is bone stock, the bike has only run 4 or 5 times a year since new, carb is always drained after every ride. It seems to run fine under most conditions, but once in a while (mainly up high) it will do this. She barely made it up this climb. We live at 2600' and this video was 6400'. It has done this before as low as 3300'. It is a very intermittent issue. Any ideas and possible fixes? It can be heard best around the 0:18 mark.