I have a 84 Nonturbo Automatic Nissan. The car idle speed is right at the dot(750RPM) with no misfiring or Rough in idle while it is on drive but I when I shift it to park or neutral my car is very rough at idle(from 750RPM to 950RPM and so on) and it sometimes misfires but not that much but one can hear it. I changed the Spark Plugs Spark Plug Wires and Distributor Cap. I checked the Head Temperature Sensor which is good and the O2 sensor but still have the same problem at Neutral or Park. I clean the throttle body with spray and made sure all the wire harness are in good tact. What are my other options. What is the IDLE-UP Sensor?