My car is a 1996 LX with 86k miles on it. When in Park, the car starts and idles fine.

When I go to shift from Park to Drive or Park to Reverse, the car will lose idle down to ~500 RPMS and begin to shudder/act like it is going to stall out, but never will. Let me also add that this problem is much more prominent with the A/C on, and will only lose a very small amount of idle RPMS with the A/C off. Sometimes the car will automatically pick itself back up to the correct idle, but it is an extremely delayed reaction. Recently, I have just been giving it a little gas to help the idle out, as it will continue to idle fine at ~800 rpms with the correct amount of pedal pressure.

My diagnoseses have been - Fuel pump / clogged filters
IACV needs to be cleaned/replaced
and from searching, I have also seen -
Possibly a symptom of oil in the spark plug tubes
EGR clogged?
Loss of vacuum pressure?

I have recently done a tune-up and replaced cap/rotor/plugs/VC gasket/upper tube seals, but the tube seals either didn't set properly or are not the problem (maybe lower tube seals).

 

TTT