1991 GMC Sierra, 5.7L - After an extended cold spell, transmission began and continues as follows: Shift from Park to D or O/D, starts out in 1st, smooth shift to 2nd at about 19mph...then at about 30mph shifts into a 'neutral' state, but if I manually shift down to 2nd at this point the tranny shifts smoothly into 3rd and will stay there as long as I keep the load on it. And it will downshift smoothly if I let off the gas. ALSO, while this is happening if I continue to accelerate up to and over 45mph i can manually shift back up to 3rd or O/D and it will stay in third until my speed drops to about 40mph. Then it falls back into the false neutral and I must again manually place shifter in 2nd to get 3rd back. And, if I place the shifter in O/D and keep accelerating it will go into O/D at about 52mph. While in 3rd or O/D if I punch the pedal it will drop to a passing gear mode aggressively, then upshift correctly. From the way it shifts from 1st to 2nd I believe I've got the shift cable adjusted very close to where it should be(but I could be wrong). I like the way this tranny feels solid and strong, but this method of shifting is for the birds. Please any ideas are greatly appreciated