1996 Ford Thunderbird Malfunction - Ignition System

Question: Engine is 3.8l non supercharged. Has Auto trans. Problem feels like spark plug misfire but it only occurs when car has shifted to 3rd or 4th gear at fairly low speeds and the rpms are under 1500. As soon as I accelerate a little, the engine starts to shake and loose power - feels exactly like a spark plug misfiring. If I step on the gas harder, it stops the problem. If I put the brake on and hold the car still while in gear then I I push the engine hard - it does not misfire at all. This seems to only happen during light acceleration and only in the last two gears and it is random. It is not related to the shifting - I can be cruising along at about 40mpg in 4th gear - no problems the when I slightly step on the gas, it starts misfiring a lot but not steadily. If I let off the gas or press down more (but not enough to cause a downshift) it will clear up. Engine has 69k miles on it. I just replaced all spark plugs with dual platinums hoping that would fix it - it didn't help at all. The old plugs were not fouled and I saw no evidence of water either. I am suspicious of the coil pack and spark plug wires but it seems like either would have a worse problem under a more stressful load condition instead of the opposite. This only occurs while slightly accelerating and only after the car is going fast enough to shift to the last two gears. I put the car in gear #2 and drove it all over - the problem never occured. It occurs with overdrive on or off also. Perhaps there is some vacuum leak caused by slightly depressing the gas pedal? I've reseated the plug wires into the coil pack - cleaned it off - checked for vacuum leaks - I can't figure out what is causing this and it's really bugging me. I don't want to buy part after part after part. Do you have any ideas? Does this engine switch to an alternate engine control mode during slight acceleration? I have a very good OBDII testing unit and it shows no errors at any time - not even with the device hooked up while the problem is actually occuring. I know the tester works as I can create an error condition and it will catch it (like unplugging the O2 sensor.) Is there a way to force the engine to run a little on the rich side so that I could help figure out if this is a vacuum leak somewhere? Maybe pulling the engine temp sensor connector off temporarity would signal a cold engine and make it run rich?

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Technican: Have you read the TSB's (technical service bulletins) for this car? Ford has issued bulletins for reprogramming the pcm or replacing parts for a variety of concerns. One of which is torque converter shutter/vibration under the same conditions you describe. If you email me I can send you this informaion- If I was continuing to experiment, I'd create a slight misfire. Does your monitor detect the misfire (or does it turn on the engine light)? If yes, it's probably not the plug wires or coils. Especially if the misfire you intentionally created is less pronounced than the one that remains a mystery.

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