Rebuilt the carb on my 8hp Tecumseh/Troy Bilt snowblower this year. Using fresh high test with Startron. After 45 minutes of run it "bogs" down, then runs up in rpm, then acts like it's going to stall. Have removed gas cap to see if there is a venting issue. Shut it off, take a 5 minute break and it will start up and run for another 10 minutes flawlessly, then act up again. Not sure where to start, any suggestions?