You are going to be pulling the pump and getting it looked at.It should be a Roosamaster pump,(unless it is a later model tractor with the Cummins,they'll have a Bosch.) Some have speed advance,and some have load advance governors.Either way there are worn parts in there that will let it creep up while it is idling.Some have a hard plastic ring on the advance held on by three rivets.They will swell and crack at the rivets,eventually breaking.There are other parts in there that can break and let it run away on you.If you can blip the throttle by hand,and it returns to normal,then starts to rev on it's own again,it's in the pump.Of course this is all provided you don't have some kind of linkage problem,but you would be able to see that.