compressor issues

What's your air pressures on governor cut-in and cut-out?
Have you done a static DOT type systems check, to check for leakdown rates? If you don't know what I'm referring to, lemme know.
Can be a few things, but possible your governor is acting up/faulty.
Also make sure your tanks are drained of moisture/water.

 

Uploader valves? Might be draining back internally and you won't hear it.

 

The leakdown I was referring to is just building pressure to gov cut-out (120 psi in your case), stop engine,
wait 1 minute & see if or how much air gauge drops...
also, do the same at 120 psi, applying & holding a heavy force on the brake pedal. Hold pedal down, without releasing, for 1 minute to see if you lose air.

A 10 psi differential isn't much. I can see it cycling quite a bit set like that. I typically try to get low end to cut-in at less than 90-100,
and prefer high end at 125 psi.

Also check for fan clutch leaking. It won't show with engine off. Have to take shop air and apply it to the line.
Also check air dryer and the signal line from the governor for clogged residue.

 

air lines off the governor probably need to be changed - soak lines with soapy water then start truck and watch

 

start with changing air/reg. one more time.

 

A 10 psi difference between cut out and cut in will have you cycling pretty rapidly, but it seems like the compressor is doing what it's supposed to. If it were me, I'd set the regulator to cut in at 100... keeps you within the 25 psi difference, and all other FMVSS specs, and the compressor clutch will cycle less. Other than that, I'd check the lines.. drain your tanks, see if you get an abundance of oil coming out. With that much use, there's bound to be some, but if you're getting copious amounts of it, then it's a good sign that the compressor is due for a rebuild. But I wouldn't expect it based on what you're telling us.