2001 Nissan Pathfinder OBDII/sensor issues

For the last 5 years or so I have had the MIL come on intermittently. By that I mean codes related to the rear O2 sensors( P0138, P0138, ect. ). I have not replaced them because the MIL will go off when driving in town for approx. 75 miles and then come back on when doing freeway speeds for approx. 75 miles. The vehicle will pass emissions while the MIL is off, so it appears that the sensor is OK and that there may be another reason for the MIL coming on intermittently. Two years ago, P1140 came on so I changed out the right intake valve timing control position sensor, cleared the code and drove at low & high speeds until all of the I/M monitors were ready. I then had it emmisions tested and it passed. Just recently, P1145 came on, so I replaced the left intake valve timing control position sensor. I have cleared the code and have driven until all of the I/M monitors are showing ready with the exception of the "Catalyst". After driving a little longer, P0138 and P0160(pending code) and P0138(pending code), came on. The I/M monitor for the Catalyst has still not ready. So, do I still need to drive a little longer at high speed for the catalyst I/M to be ready and do I have O2 sensors that are defective or is there some other electrical problem? I have disconnected the engine grounding leads and cleaned all surfaces several times as the shop manual has suggested. I need to have the vehicle tested before the end of this month, so I'm hopeful for a fix. Something else I found out from AllData is that if it is determined that the O2 sensors are bad, that after installing new ones, the ECM must be reprogrammed so that the O2 sensor heaters come on slower so that the bulbs do not fry as what may happen when the current heaters come on too fast. I live in Arizona and am wondering if the reprogramming is necessary, as we don't get alot of real cold weather. What is your opinion? I wish I had access to a ConsultII analyzer; do you know of any aftermarket analyzers that come close to it?