I have an old JD 317hp lawn tractor. It has a newer(11years old) Kohlor 18hp engine in it. I have had no problems until this summer when i noticed the engine a little hard to start. During the hot summer months the engine has been acting up. After the tractor has been working with the grass cutter for 25-30 minutes the engine starts to surge much like it would if it were running out of fuel. It will get progressively worse until it stalls out. I have changed plugs, fuel and air filter, and nothing seems to be working. It appears to act up when you load it up. Could this be a govenor issue?