A mystery even the car talk guys might be unable to solve

Ok. So...
A long time ago, in a city far far away, overnight the air became frigid-the years first frost glazed cars's windows. The following morning, trouble began. When the car was in park, the idle would be
somewhat rough. When it was in drive, the idle would be more rough. The roughness would smooth when the car accelerated, but the car would not accelerate without hesitation. I did a couple of things, and the problem went away; however, it's unclear if anything I did had any effect. I changed a spark wire, and dropped in the tank some dry gas.
But the problem came back. I don't remember the relative temperatures as keenly; I do remember it was cold, and I do remember that the problem again precipitously went away after I did something. I took off the air filter housing, checked my air filter, and replaced its housing; the car had been running for a while.
The problem came back. I don't remember the temperatures, but I do remember it was cold. Without doing anything, the problem went away by the following morning. Then it came back. I changed all the spark wires, and replaced the filter-which I think I may have been improperly replaced the aforementioned time I checked it. I think it had been replaced cocked, not forming a seal.
The problem went away, again precipitously, but only after the engine did more driving.
In Buffalo two nights ago, the temperature, after having been unseasonably warm, dropped dramatically, and it snowed. Last night, the temperature still being (more usually) cold, the problem came back.
And again, it came back precipitously: After the car had been sitting for perhaps two hours following a short drive, soon after I began driving it back home, the car suddenly began vibrating at low speeds, vibrating while stopped at lights, and from a stop the car began accelerating hesitantly.
I had someone check the sensors once. They said that they were fine-that they had plugged in a diagnostic tool and read the sensor outputs, and that they were correct. But this same person was retarded, so he could have been wrong. (When I drove my car away after he had done his work, I noticed the problem was worse, and then noticed he forget to reconnect two of my spark wires.)
There is no check engine light: there never has been. When the problem is active, there is pungent, more voluminous exhaust.
My car is a 1995 Chevrolet Lumina Sedan, 3.1 V-6.
Thanks alot, Sean.
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Sean:
Have you checked the engine temperature sensor? Not to be confused with a temp gauge sending unit BTW. I don't know where this would be on a 3.1, but when they fail, the engine stays in open loop and one of the give away clues is a rich running engine - you can smell it. A scan while it's acting up will show whether it's in open loop or not. O2 sensors can also contribute to an open loop situation. Don't get too hung up on the absence of codes - that can happen.
The other thing I've been prey to with this type of engine is a bad coil pack. It can sometimes be troublesome locating which pack is bad, when they are intermittent. You can always try changing out one coil at a time until you fix the problem - if that's the problem.
--

-Mike-
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