first, make sure it is not starving for fuel.. they will 'hunt' when fuel supply is inadequate... engine will start to stall out.. throttle setting makes gov pull back and rev tractor up.. this upps manifold vacume pulling a jet of fuel in, causes over rev, gov relaxes, and then the oscilation starts all over again.
look at your hand throttle rod to gover nor.. notice a small spring it passes thru.. then notic ehte gov to butterfly connection.
you can test it by disconnecting gov to butterfly aand see what she does manually controlling throttle with hand on butterfly.. that way you can see/hear if she is stalling out, then see what gov does by observation.
ps.. got the service manual?