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Since it's worst with A/C on at idle, I'd look at 2 things:

1) Charging voltage at idle; and charging system and battery in general.
2) A/C compressor starting to bind and drag. As IC suggested, tell us the history of the A/C system service.

One very rare thing to check is that the cooling fan motor windings are shorting out. With engine off, unplug the cooling fan motor and measure resistance across its contacts. Should be in the range of about 2.4 ohms. If it's about 1 ohm or less, the fan motor is shorting out and dragging the idle down due to the electrical short.
 
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You say you replaced the oil pressure sensor and it still reads zero. Are there one or two wires to the sensor? If one, then the ground for the sensor is through the sensor threads to the engine block. If you used teflon tape or any similar sealant, then you have electrically insulated the ground from the block and it will read incorrectly. No teflon tape should be used.
 
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One thing I think we all missed, and it's basic, is to do a full tune-up. These engines eat ignition rotors, and so they need to be replaced every 15K miles. They also burn the forked tips of the ignition wires inside the cap, and so at 15K miles you can remove the wires from the cap, turn them 180 degrees so that the other side of the fork faces the rotor. Then at 30K miles, the wires need to be replaced.
So, replace plugs and ignition rotor at 15K miles, and at 30K miles, replace the wires and distributor cap. If you haven't followed these intervals, this could cause stumbling at idle. Once I missed the schedule, and at 20K miles, I had a sudden no-start. Took me 2 days to diagnose the burned ignition rotor, and once replaced, it started immediately and was smooth.
 

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Need some info on the Neutral Safety Switch (NSS). In the manual it states the NSS is an input to the computer used to control fuel, idle, and timing. I compared diagrams in the original OEM service manuals and Chilton's manual and they are the same. Round connector with three terminals. The neutral safety center terminal is normally closed and shorts to the casing for P and N for the starter relay. For neutral safety it should open on R, D, D1, and D2. The outer terminals (1,3) are for the reverse lights and is normally open unless in R. This is what I summize from the manuals and if tested should read as only switches (open or closed) with no resistive states in between and need to know if I'm correct so far. My old NSS reverse light switch works fine if I press the center tab, but the neutral safety contact is constantly open. What has me confused is, if my old neutral safety contact is constantly open, my car should not start, yet it does. To add to my confusion, the new NSS I bought, TE121, the reverse contacts seem to work if I press the center tab, but the neutral safety contact is constantly open and no matter how I actuate the device (does nothing). This leads me to believe the new NSS is defective right out of the box if it does not change. Unless the NSS is not being used as a neutral safety which means both old and new switches are working. So I need un-confused and expect there was a design change sneaking in the background.
I don't think you're interpreting the function and states of the switch correctly.
The center wire goes to ground. One of the outer wires goes to the backup light switch and the other to the starter relay. The switch only matters to starting, and not running. It has no effect once the vehicle is started.
 
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