thanks Grandpa,
well, one of the first things i would check is the control handle and switches and wires inside it. If the handle is worn it might cause the initiate circuit to disengage when moving the handle. Get a vom meter and hook to the switch circuit and monitor that while moving the handle and see if it drops out at any point while moving it. If your platform controller is what came on it and hasn't been replaced or coverted to a new style then it should have a travel potentiometer, you can check that and make sure it doesn't have any dead spots in it, that would certainly make it stop erratically.
another thing that comes to mind, you said this came from an Air Force Base?
Some units that operated around airplanes had a special kit installed on them that would disable the travel if it got too close to the plane, some sort of arms or sensors around the platform railing? Does this unit have anything like that on it? If so this could be causing the travel issue.
Then of course you can put a vom on the fwd/rev valve on the main manifold block and check the travel signal there and see if it is dropping out erratically.

Is an alarm sounding when it stops?
that could be the tilt sensor causing it to stop, they have been known to go bad and cause issues like this also. The older style ones had an acutal pendulum inside it that contacts a ring and if it shakes too much it will stop the unit and sound a tilt alarm or blow the horn. Is any of this happening?

these are a few things to look at.
if you want any documents on it you can go to JLG's website and get their service and parts literature at no cost and download them. The service document your looking for according to your serial number is 3120691.
As for parts you can use the interactive section and input the serial number and it will take you right to the parts section.