no 4th gear until 52mph

I have a 1997 chevy p30 with a 4.3l engine and the 460LE trans. 1st, 2nd, 3rd and reverse gears all work fine but I never get 4th until 52mph and some times not at all, at those times if I shut the engine off and restart it will go in 4th at 52mph. shift solenoids have been replaced pcm replaced but not the chip. and a sensor on the trans I think called a shifter position sensor was replaced. I am stuck on this one and it is costing me loads on these misdiagnosis. please help, thanks

I have a 700 r4 and it has a cable hooked to carb and when this was out of alignment I had your problem hope this helps Ben