Car randomly shuts off on me

Where do I begin?
I currently am driving the car listed above and unfortunately it has been my second one in the last three months! At the beginning of the year I bought a 1994 Park Avenue with its own special set of issues which ended in a rod knock in the engine after a month of having it. In the short time I had it a constant problem would occur in that the car would idle bad and shut off on me while I was driving it, at stops, and even while I leave it sitting running. I replaced everything you could think of battery, fuel filter, spark plugs, the entire exhaust, vacuum hoses, intake manifold, EGR valve, air filter, I could go on. After putting all my time and money into that car I did not want to let it go so I bought a second car (same as the first) used one and kept all the brand new parts off the first one to put on the second. So now it has been about a month in with my second car and now it shuts off on me too! I bought the car originally knowing there was a small exhaust leak other than that the car ran great. I am still in the process of swapping out parts so I could still have the solution my car needs I just have not gotten around to replacing it yet.
Two or three days ago my current car starting shutting off only when I am driving it as well as the service light coming on/off but up until yesterday the service light is now always on. The car still runs like a champ, no rough idling. But I have noticed when I give it gas and let off the pedal going about thirty to forty mph the rpm's drop and that seems to be when my car shuts off. When this happens the car will not let me restart it in neutral I have to let the car come to a complete stop, put it in park, turn the car off and it will start up every time. This is not fun and not to mention dangerous! Too afraid to try to take the car anywhere and I cannot just take it anywhere to have it tested so I turn to you internet!