I'm having a problem that seems to have happened recently, The car will start O.K. but within seconds begins to idle very rough. The tach will swing from 800-900 down to 300-400 and the vacuum guage will keep time with it(normal vacuum at the higher rpm and almost none when rpms drop) and at times will stall completly. If I give it a little gas and keep the rpms around 1000 it will hold the engine speed but still sounds and feels like its missing on one cylinder intermittantly. While driving the car seems to run fine anytime I'm on the gas or rolling at speeds >10mph. When I stop I'm back on the carnival ride.
What I've done so far:
Checked for codes - None on socket 2 or 6
Checked for vacuum leaks - Couldnt find any but according to my test gauge I've got low vacuum at idle @15-16 instead of 20 like it used to have
Recently replaced Cap & rotor(bosch), Wires (IPD), Plugs(NGK std)
TB & IAC Cleaned @ 2-300 miles ago
Swapped Iac With another used unit - no change

What I've considered:
RPM sensor?
Fuel injection resistor?
Power stage?
Knock sensor?

I'm hoping for some advice on what or how to proceed before I start guessing and spending alot to replace parts Thanks for reading my overly wordy post, and any help you can offer.




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