Injection pump maybe???

I have a jd 4050 I know wrong color but it has a roosamaster pump. Anyway the problem is the governor doesn't seem to respond as quick as it should. It goes to far below and above set rpms without responding. The real noticable thing is when you go down hill it holds back good and then just starts to climb in rpms, like it loses its hold back. Engine has plenty of power, very little blowby, no oil consumption to speak of and starts excellent down to 0 farenheit. It actually starts way better than it should in cold weather. I'm not complaining but never had a Deere that starts this good. Is it likely internal pump issue or a timing thing. Thanks just looking for some advice before I dig in.

The 4050, is a great tractor, I sure wouldn't trade it off for a simple problem. Yes, the injector pump needs to be rebuilt. I can't remember the exact cause, but something is getting hung up in the pump, not allowing it to respond properly.

I've seen a few DM pumps have the weight retainer ring slip, so then the governing gets very sporadic. Minor changes, it works ok, big changes, it just slips and doesn't control anything. Pump issue for sure!