Those autos are usually pretty bulletproof. I've replaced a bunch of the PNP switches, but only 1 tranny which had been run low on fluid and really abused. I don't have a sure fire test for the switch, but usually I find that if you start the car, warm it up then restart it, the problem clears up until it gets shut off and cools down. Do you get a "check trans" light on start up?