2001 chevy 3500 2wd 8.1L gas engine. Coded with speed sensor output error. Changed speed sensor and problem cleared for awhile. Now it is back and more frequent. Problem can be "cured" by go to park, kill ignition and open door. This resets the circuit and transmission operates normally upshifting thru the gears. Decelerating thru the gears locks up on second and stays there, also looses reverse. Any ideas from simular problems??