hobart welding machine problem 1

i have a hobart mega arc 300G, 1984, ford gas engine 2300, 4 cyl. machine. When i am welding with a 1/8" rod, sometimes the machine will speed up and the heat will change like i am burning a 1/4" rod. the only way to stop it is to turn the machine off and let it cool down awhile. then when i start it back up, sometimes it will work right and sometimes it will not.i have had the dios checked and it checked out ok , as did the load test, and the generator test. i have had new slip ring brushes put in and seated and a new 1"x1" rectifier box. i have also put in a new govenor and replaced the timing belt. can you give any advice? thanks, john moore