Are you hitting the governor at the end of the straight away or way before it. Gear it to hit the gov just before the corner. I just put your pulley/sprocket info in the ec distributing technical mower speed page. you are at 6.875 to 1. The last track I ran on that had a 100 ft straightaway I was right around 6.5-6.6 to 1 ratio in 3rd or 25-25 gear set in a 700 series trans. I was hitting the gov just before the corner. This setup seemed to work good for me. As for the studder, sorry I cant help you there. Maybe carb issue?