I have a 91 Firebird with 160k miles V6 auto. Yesterday I drove my car fine for about 50 miles came home then I went back out to the store on the way she shook and died. I start her up and she would shake and die, shake and die. I let her alone for 5 mins, then I turn her on and she would sound ok, then I gave her gas(in park) and she would die so i let her cool down. 6 hours later I went to bring her back home from the store I left her at and she started up and drove home (about 1 mile) I came home and popped the hood to check things, all seemed normal. Spark was ok, reved the engine no problems no dieing.
Today she died at the same store same situation, after reading a few things I tried the car with the TPS unplugged, it stayed on but would die if I gave her gas. I checked the error codes and got a 12, I read that could be several things. I was able to get her home in "drive" without giving her any gas so was driving about 3 mph, and the check engine light was on and brake was very stiff but usable. This happened to me about 2 years ago (minus the stiff braking), did this for almost a week, I had Midas look at it, replaced fuel pump, EGR, fuel filter, nothing, they couldn't figure it out. I towed it home and my dad saw that one of the spark plugs seemed to be bad, replaced it and it ran fine(I do not know if the spark plug was actually the root of the problem) for 2 years now.....any help would be greatly appreciated.


UPDATE: So today I add octane boost and lucas fuel injector cleaner, replaced fuel filter, air filter, and pcv valve. Drove it cold okay, after a few minutes problem came back I went under the hood again, it seems that the 3 cylinders on the driver side are not firing. I unplug a cable from the passenger side and it stutters and dies (so those 3 are working) and when I unplugged all 3 cables (at the same time) on the driver side there is no difference in the car, still running rough but not dieing or stuttering as bad as when I unplug one of the passenger side.