I had the exact same problem it took me a whole summer and ford a converter, two transmissions, computer, and speed sensor to figure out whats wrong. Your wires are throwing your speed sensor off. Either you just changed your wires(hi performance type) or your wires are bad. But I'm almost sure of it. Your speed senor thinks your car is going 50 mph at idle. Thats with all the shifting off idle and low speed and such. But stock replacements back on it.
