I Have a D-10 military blazer (cucv) I installed a new Vacuum pump, trans modulator and a shift kit (really just higher pump pressure spring) The truck shifted at 20-23 mph and 30-34 mph. Good firm shifts and no problems. One night it started not to up shift till 45 mph and then at 60 mph. I checked the vacuum pump and it was shot. installed a new pump and still the same thing. when the trans is really cold, sitting for a couple hours it will shift fine one or two times then back no the no shift. I replaced all the vac lines and there is vacuum at the modulator and it moves when vacuum is applied. All seams to be working on the vacuum side.. what else could it be?