2001 Chevy Blazer transfer case

transfer case will go into 4lo from 2hi on its own. this occurs after a brief run on open road and coming to a stop.It does not mater whether it is n drive neutral or park, after some fussing with the buttons it will go back to 2hi. it is a np236,auto or pushbutton select style .drained oil and check for metal and debris,found none. replaced with the gm fluid .can hear the motor and forks engage when assistant pushes buttons. removed motor and manually shifted stub shaft,it solidly engages the ranges and doesnt feel loose. checked connection and inspected wires for damage.pulled atc computer out and found corrosion on 2 pins in the computer termination plug and corrosion on the power supply plug on one pin.cleaned thourghly and reassembled.it actually made the problem occur consistently. inspected dash switch,nothing obvious and it does function. I have read every idea I can on the internet and gone over the pieces many times. in ther driveway with an assistant pushing the buttons it will shift and stay and have even put in the air and live tested it. back on the road however the problem is consistent. Any help is greatly apreciated. thank you Mark Guin