I have a 15 year old Generac 12/15kw standby generator running on natural gas. Recently it shut off tripping the overspeed alarm. I adjusted the mechanical governor and it ran well for awhile, then it did it again. Re adjusted it and it ran fine until this last time. I got it to run briefly at about 22hz. Then 28, then 30 something. I got it up to 48 hz and it tripped the overspeed alarm again and shut off. I did not have an engine tach on it, but it did not sound like it was running fast. And there it sits. Anyone have any idea how to fix it?