general problems of the 1997 Nissan Maxima

My 1997 Maxima has had the check engine light periodically come on for the last 4 years. The obd code is for the speed sensor (p0500), I have the speedometer drop to zero occasionally and believe there is an issue with the circuit. I have changed the sensor four times and the problem always returns. Testing the sensor shows it is functioning normally. To get my car inspected, I reset the code and it eventually returns to normal and I can pass the emission test. This year, I have incomplete codes for egr and catalyst monitor. There are no codes pending and I check the status daily. I have about 500 miles on the car since the code reset and it continues to register incomplete. I am afraid that I am going to get fined for the expired inspection. Are there any known issues related to this problem?.