Grand Marquis part throttle misfire

My '92 Marquis LS has a part throttle misfire. I tried running some fuel system cleaner through and giving it run down the highway but this didn't make any difference.
The odd thing is if it is cruised at 120 KMH there's no misfire but if I drop back to say 80-90KMH and then accelerate back to 120KMH the misfire starts again. It also misfires in city driving again on part throttle acceleration.
It has new plugs and air filter. Does anyone have any suggestions as to what I should look at next?
Thanks
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Throttle position sensor maybe?? They can develop dead spots over time.

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Does it still have the factory original plug wires on it? If so, replace them with some good quality wires. If you pull each wire and inspect the lower part of the boot you may be able to see where the spark has been jumping through the boot to the head. It's been a common problem. Bob

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Plug wires. Typically this is not a "fuel injector" problem as you found out. Most likely the plug wires are the problem, the clue is the problem seems to happen at high loads what your post seems to indicate.
.

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Check for vacuum leaks and a malfunctioning EGR. Replacing the cap and rotor may help, but is very unlikely to be the problem... Just something you ought to do to complete the tune-up, unless of course a '92 has COP ignition, but I don't think it does.
Cory
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Thanks for all the replies. You've given me some good ideas. I'll go through the ignition system and see if I can find any plugs that look they are not burning fuel cleanly then see if the lead to that plug is bad. After that I'll check the voltage on the throttle position sensor then then check the EGR etc. Well at least now I know what I'll be doing next weekend :)
BTW does anyone know if these are any good for reading error codes?
http://www.midwayautosupply.com/detailedproductdescription.asp?3829
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