I found the following description in the FAQ. It matches exactly the problem I am having. I have on other thing to add though, I heard a strange noise coming from what sounded like the trunk sounded kinda like a string being plucked. It occured about 4 or 5 times then the car started to run correctly again. Has anyone had and fixed this problem before? I have also incled the diagnosis although niether seem to be definitive.

"High-Speed Misfire, Drop in RPM. [Symptom:] The engine idles and runs very smoothly until the engine is pushed to RPMs exceeding about 3,100 to 3,200 RPM. Once those RPMs are reached, the engine begins to shimmy, shake and bog down such that RPMs (if the accelerator pedal is held down) actually drop down to about 2,200 RPM. Only when the 3,100 RPM "wall" is breached does the problem manifest itself. The problem occurs regardless of whether the car is being driven or is sitting in neutral. This is an 89 745, 230F with an engine from a 93 940. I have swapped or replaced the following items since engine replacement (and since discovering the problem) without any noticeable effect occurring re. the 3,100 RPM wall problem: cap and rotor; spark plugs; spark plug wires; all fuel injectors; fuel pressure regulator; in-tank fuel pump (new); fuel filter (new); and crankshaft/RPM Hall Effect sensor(new). I also tested the fuel pressure under static and load conditions. Under static conditions the fuel pressure on the high side of the fuel rail is about 45 psi. Under load conditions (i.e., engine running) the fuel pressure is about 40 psi, and does not decrease when the 3,100 RPM "wall" is hit (it actually increases a little bit to about 42-43 psi). The "Check Engine" light is not on. I disconnected the air mass meter when the car was idling with no noticeable effect on performance occurring. "

"[Diagnosis 1:] Are you sure the misfiring is dependent on the RPM and not the throttle position? I had a similar problem with a Ford, and it turned out to be a worn out throttle position potentiometer, at certain positions it would hit a bad spot and the ECU would shut off the injectors. You said that fuel pressure increases when it starts to miss, that suggests that the injectors shut off or at least not open as long as they should, so something suddenly starts telling the ECU that the engine doesn't need as much fuel as it actually does.
[Diagnosis 2:] Perhaps, the fuel pressure regulator. Had the one on my '87 745TGA go west on me. The darn thing would hardly hold a hundred with the throttle floored (that one got a laugh from the techs--they thought I was kidding). But he's right, the switch sometimes goes bad, too, though I thought I remember you mentioning you had gone to full throttle and it still was bad. They sometimes use electrical contact cleaner and a good stiff wire brush on those units (though that happened more often with the '83 245 than with either 74x)."

Thanks In advance,
Doug