Why is my EGR position sensor melting?

1989 Ford Taurus LX wagon with 3.8. My car has recently destroyed two brand new EGR valve position sensors from two different suppliers (BWD? and Echlin) by melting the bottom of the case. Prior to that, it had destroyed two original parts as well. Right now I have a rubber tube in place of the sensor, with a bolt to plug up the hole, so the engine runs, but it does not have a signal from the EGR position sensor which is causing the MIL on. I have tested the EGR valve with a hand held vacuum pump used for bleeding brakes, and the valve with open with as little as 5 in Hg. When the engine was warm and running, I could feel vacuum to the EGR valve, so as it is, my ECU is not getting a signal from the position sensor, and is reacting by opening the EGR at operating temperature. The OE rubber tube to the sensor has been scorched during the melt-downs, but right now, my temporary vacuum line with a bolt in it has been staying flexible. I had been driving around for a while with this set-up before purchasing a new sensor, because I wanted to see if my temporary tube would get scorched as well - it didn't, so I bought a new sensor, and the problem came back, even after clearing memory codes. I had also tried installing an original sensor sourced from a junkyard, and with that in place, the engine ran so poorly that I immediately went back to my temporary plug set up. I would think that hot exhaust gases are melting the plastic case, but what could cause that? A lean A/F ratio? Not enough EGR flow to cause high combustion temperatures? Fuel economy is fairly normal, although a little low. It seems like the engine runs OK with the EGR sensor port plugged up, and that the problem lies solely in the sensor, but replacing it hasn't seemed to fix the problem.