I just got back from a elk hunt out west. I used my Grizzly 550 on the hunt in altitudes 7000-9000 feet. After running a few miles it would start missing and stall out. I would turn the key off and back on and it would start fine like nothing was wrong. When I would pick up speed it would start to do it again. After the 1st day of this I changed the spark plug. It was dry. The 2nd morning I probably traveled 8 miles before it did it again. After it started it was a struggle getting it back to camp. All of this didn't start until the 3rd day I was out there. Last year it didn't act up the entire week I was there. If it matters the only thing I burn is 91 premium non ethanol fuel. Has anyone else dealt with this?