My 1999 lincoln navigatol stalls on me, i've rplaced fuel pump, i can not go over 20 mile per hr.

Is the check engine light on? A bad speed sensor or possibly another sensor could keep the vehicle from up shifting or limiting speed to bring it home in "limp in mode" Possible bad catalytic converter.

I have one cause in mind, because it's often so obscure and difficult to diagnose, yet easy to fix. It's afflicted several cars I own. There are numerous possible causes, of course, like a coil that isn't feeding enough energy to the spark plugs, a choke that's stuck, a fuel pump that's not pumping, an electrical malfunction. But the one that comes to mind is a bad "sock' in the fuel tank. This "sock" is a toothpaste-tube-like fine screen in the fuel tank that is supposed to prevent small particles of junk from entering the fuel lines and the fuel pump. Over time, the pores of this thing can become absolutely restricted--I don't know whether it's a reaction to the gasoline, or whether it's a natural result of the material aging, or what. Result: Hardly enough fuel gets to the engine for it to run. And, of course, the faster you go, the more fuel you need, and the more noticable the restriction is. I've had this happen to two of my Fords, one of my Chevies, and a Buick that a friend had--though, at the time he had his problem, I didn't realize what it was, so we went through the usual routine of replacing fuel filters and fuel pumps. I'd say it happened to my Lincoln, too, but the sock in that car actually disintegrated and wound up being jammed to the carburetor's filter by the fuel pump. Yet in the case of the Lincoln, I couldn't get past 20 mph until the problem was solved by removing the tank, flushing it, reinstalling it, and installing an in-line filter between the tank and the fuel pump. In your case, if a sock is really the problem, I'd remove the fuel sending unit in the tank, chop the sock off, and install an in-line filter. Take this course if you've determined that the car is starving for fuel and you're sure the fuel pump isn't bad. One other possible symptom of this sock problem is observed when the car runs decently in cooler weather and starves for fuel in warmer weather.