automotive engine

Mercury Sable Drives Itself
Q. I own a 1990 Mercury Sable, with relatively low miles. I am the second owner and the car has 70,000 miles on it. It has a 3.8 liter 6 cylinder engine. The problem I've been having is recently my wife was driving the car and it started racing.

It was accelerating all by itself, she was very scared and barely made it home. I thought the pedal may have been stuck or that the linkage was stuck and dirty. I sprayed some solvent and tried working the linkage by hand to free it up. No help at all. I then noticed a module, (which I later found out to be a air flow valve), I took it off and noticed a difference. I thought that I solved the problem.
I went to the local bone-yard and got an exact copy of the part. I replaced it and the idle went back to normal immediately. I didn't even test drive it, I thought it was fine. The next day my wife goes to drive the car and it won't go any faster than 10 miles per hour. I get behind the wheel and the same thing. Then I pushed down hard on the pedal (The pedal is very stiff now) and the car did accelerate. BUT, when I let off the gas, the car jerks and sounds like the gas flow is being interrupted or something.
I then looked at the vacuum hoses to see (although I really don't know what I'm looking for) if there was any old, soft, or broken hoses. I couldn't find anything. BUT, what I did discover was that the gas pedal is very hard to press when the car is actually running, BUT when I shut it off, it's very free and easily depressed as before the car when it was running fine.
This might be the key to the problem but that's where my talent ends, I don't know what to do now. I'm HOPING that you can help me resolve this problem. PLEASE help! Thanks so much!
Derick
A. I think what you replaced was the Throttle Air Bypass Valve. The Idle Speed Control/Bypass Air (ISC-BPA valve controls idle speed by regulating air that bypasses the throttle plate.
There are two parts to the ISC-BPA. The bypass air valve (BPA) and the idle speed control solenoid (ISC). The BPA valve is thermally controlled and functions only during cold engine conditions (below 140 degrees F). The ISC valve is controlled by the electronic control assembly and operates under all engine speed and temperature conditions.

I think you replaced the right part with the wrong part. The valves may look the same, but their operational parameters will be different. This may explain why the gas pedal is hard when the engine is running and normal when the key is off.