1995 Altima Speed Sensor - Code 720

Well, this forum helped me through my distributor probem a while ago, now I need your help again. My Altima is a 4 cylinder with 110,000 miles. The car had some shifting problems over a year ago and code 720 came up on the scanner. I replaced the speed sensor (Nissan also calls it revolution sensor), which is on top of the transmission case near the firewall. The car ran perfectly for several months, then it started shifting erratically, sometimes wouldn't shift into 4th gear, then it would be okay for for a few days, finally it wouldn't shift out of first gear and my wife drove it a couple of miles in first gear to get home. Scan showed code 720 again. I replaced speed sensor again (a dealer only part). The car ran fine for probably 4-5 months, then last week the car started to dowshift at 35-40 mph and wouldn't stay in 4th gear. After stopping at a stop sign a few blocks later, it wouldn' shift out of first the rest of the way home. The next day, I took it out and drove it for 20 miles with no problem. A few days later, it wouldn't shift out of first again, code 720 is back again. Every time I replace the speed sensor the code goes away for a few days and then comes back. Can something be causing the sensor to fail, like an electical short or harness? How can you tell if it's the harness, and how hard or expensive is it to replace? Any ideas?