1999 Nissan Altima Nissan Pros, thinking caps please. Rumb

I've been having a strange problem that, after three trips to my mechanic and one to the Nissan dealership, has yet to be correctly diagnosed. It just won't go away. It's happened two winters in a row and usually goes away once the weather warms up, but not this year! It's now about 70 degrees out and the symptoms are still present, just not as bad as when it was freezing outside. Here the symptoms, they all happen OCCASIONALLY - not consistently:

*Stalls when put into gear
*Rumbles and shakes when stopped at red-light
*Barely accelerates when gas pedal is first pressed, goes really slow for a few seconds and then jerks into normal speed
*Idles high when first turned on
*Idles low in reverse (or stalls as stated above)
*Occasional smell of gasoline inside the car

This same thing happened last winter, and was fixed after a few tries until THIS winter...now it's started to do it again. It seemed to stump my mechanic last year, they changed an o2 sensor, and I feel like maybe he just got lucky and the problem went away until this year. It does it most in cold weather, and usually stops after car warms up or is driven a mile or so.

I thought I had an intake manifold gasket issue, because they seem prevalent on this generation of Nissans, but the dealership said the intake manifold gaskets are perfectly fine! Ha!

Anyways, I currently have a po325 code (Knock Sensor code) on it, which both mechanics seem to have ignored. I'm really at my wits end here. Any help would be appreciated. I'm either going to have to sell this car for next to nothing and try to come up with money for a new one, or fix this issue - I'd rather fix this issue.

This is clearly not a simple or everyday problem, within the past few months I've had a full tune-up, detailed fuel injection cleaning, and have had substantial testing (diagnostics and other tests that I can't remember the names of, I know one was a bass test or something?) done at my mechanic and at the local Nissan dealership. Please don't send me an answer telling me to check my filters and such, because that's not what it is.

***Id like to add that this car is extremely well taken care of (except for that dirt you see on the dash). Ive kept up with all maintenance and have gotten pretty much all recommended work done. Im peeved!***

You can check out a video of the car during a freezing morning about a month ago here: http://www.youtube.com/watch?v=Majkr2qTXOE

Feel free to ask me any questions.