I have 1941 9N which I have owned for about 30 years. This is a working tractor and its primary purpose is plowing snow. Parts have been replaced over the years as required and the tractor generally runs well. Over the last few months I have noticed after I work the tractor for a half hour plowing the tractor starts to surge to the point of wanting to stall. If it shuts down and u start it after 15 minutes it then runs fine. My question: could this be the coil. It seems to be related to heat as it only happens after working the tractor for a bit. Any ideas would be appreciated. I have replaced the coil a few times over the years but have not experienced this type symptom