1996 ciera shifting/speedometer problems
 

I deliver mail with my 1996 Ciera. Recently, it's developed some problems that the symptoms said was the speed sensor so I replaced it. It sometimes does these things:
won't shift out of second, speedometer stops functioning, shifts normally but then seems to go to neutral. Problems persisted after I replaced VSS. Sometimes I can shut off vehicle and then it works normally for a while. I consulted local Olds dealership. I replaced the ECM and checked continuity of wiring from ECM to VSS. All is good. I've tried to reset the ECM by pulling the fuse. I can let the car sit a couple of days. I can take it out and all seems to be normal as it shifts well all through the 4 gears. I can park it and the next time I take it out, the problems are there again. It's become a puzzling obsession. Any ideas??