Chrysler Repair: 93 Chrysler Imperial 3.8 Hesitation, muffler shop, chrysler imperial

Question
I have a 93 Chrysler Imperial w/ a 3.8. that has around 79,000 miles on it. It runs and drives fine around town. However, at highway speeds (around 65 to 75) it sputters and jerks and has hesitation issues. When you bring the vehicle back down below 65 it smooths back out. This has me and my usual mechanic stumped. I just purchased this vehicle in February and had my mechanic check it over. I purchased it from an elderly lady who couldn't dive any more. (The vehicle only had 77,000 on it when I purchased it) He did have to replace the serpintine belt. One more thing this car did sit idle for a short period of time before I purchased it. Any ideas?
Thanks!
Philip McGhee
Durham, NC

Answer
Hi Philip,
I would always begin by checking for fault codes:
"on-off-on-off-on and leave on" the ignition switch within a period of 5 seconds or less. Then the check engine light will be "on" but begin flashing, pause, flashing, pause, etc. Count the number of flashes before each pause and keep track of those numbers in your head. After it stops write down the numbers in the order they appeared. Group them in pairs to form two digit numbers, the last number should be 55 which is the code for "end of readout". Repeat to verify that you have them correctly.
You can get a translator at www.allpar.com/fix/codes.html
Or write me back with the code numbers and we'll discuss what to do about them, if any other than 55.
Other reasons for the high speed miss are:
a partially clogged catalytic converter
inadequate fuel supply due to clogged filter or weak fuel pump or clogged injector
spark plugs or wires arcing, faulty coil pack
The catalytic converter can be tested,
at a muffler shop that is to be trusted for honesty.
The fuel pressure can be tested with a gauge by the mechanic. The filter is under the car, near the gas tank outlet.
There could be water condensed in the tank from the period of non-use, so you could try adding a can of water dispersant to the tank and see if that clears up over time.
The mechanic can check the ignition possibilities I listed.
I'll be pretty busy until Monday afternoon but you can write back via the "rate response" tab and I'll get your message that way.
Roland