I have a 1984 F 250 6.9 C6 trans. It shifts @ 35 MPH to 2nd and 55 mph to 3rd. I replaced the vacuum pump, vacuum regulator valve, and vacuum modulator. Pump first, it was bad, no breaks/ heater etc... that is all working fine now, shifting was still bad. Modulator next, poor shifting continued I adjusted the screw in the inlet tube and eventually took it all the way out to see what would happen, it never changed the shifting problem. Although it does down shift @ 45 mph which the stock modulator did not do. Replaced the VRV no difference in the shifting Checked all hoses, swapped the two hoses on the valve top for bottom no difference. I bypassed the VRV altogether and still no Difference even with direct vacuum to the modulator! ANY IDEAS!!!!!!! I'm tired of playing with it. It shifts firm and seems to be fine mechanically just very uncomfortable to drive like this!