I think that if it was a fuel supply issue, it would get worse at higher throttle settings due to greater fuel demand. Air in the line would cause injectors to missfire or deliver less fuel resulting in rough idle or cylinders cutting in and out.
Unfortunatly it does sound like a governor/injector pump issue to me. The hand control you use is not really a throttle, but a speed selector. The actual throttle/rack control is moved by a centrifugal governor inside the injector pump. The spinning governor wants to close the throttle. By advancing the control lever, you are adding spring pressure to oppose the governor and open the throttle. As RPM increases, the spinning governor gains energy to fight against the spring and close the throttle again. For a given hand lever position, there is an RPM where the governor centrifugal weight is equal to the spring weight you are applying with the lever. The Jinma 200 series appears to use spinning ball bearings as the flyweights of the centrifugal governor. These press outward and force a cone shaped cup with greater and greater force as the RPM increases. Here is a page with pictures(about 2/3 down the page) of that portion disassembled courtesy of Tractor Outlet.
Service Bulletin from Tractor Outlet Jinma Tractors Loaders Backhoes
At idle, the spring force applied is light. Something such as a broken spring or spring holder could be randomizing this applied force or causing it to internally jam so the spring force cannot return to the proper low level. It could also be something with the sliding cup assembly not allowing the governor weights to come out all the way and set a 900 RPM low speed selection(idle). Lack of oil some time in it's past and excessive wear, dirt in the mechanism or poorly machined part to begin with could be interfering with this process at low RPM. Since the forces involved(spring and centrifugal) are less at low RPM, it takes less to interfere with them.
