I have a 1972 c10 with a 454 and 400 turbo and 3:42 gears. The engine is fairly stock, stock cam, headers and carb.the 400 turbo is brand new rebuild (6 months old). Stock converter mild shift kit. My problem is that no matter how hard or how easy I run it it shifts from 1st thru 3rd in 75 ft "short shifts". Now, if i shift it manually it shifts nice and snappy as it should with a tire chirp into 2nd and a firm 3rd.I have traced and replaced the vacuum line to the modulator, changed the modulator using two different brands, changed fluid and filter, PLEASE , your guys help would be greatly appreciated!!!!!!!