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1989 Mustang GT 5.0 sometimes bucks when letting off the gas when coasting

Title says it all but only happens a few times a month. Car has 111k on it had it since brand new all new paint etc and driven about 5k a year.
Only thing replaced were a water pump, starter, and complete clutch at 90k due to a bad throw out bearing and not to abuse, spark plugs / wires
rotor and cap and fuel pump. ( not bad for a 24 year old car ).

I have had the EGR vacuum line plugged for the past 10 years due to it sticking open, runs fine on 87 pump gas and no pinging even at WOT.
I have cleaned the MAF and IAC several times. You can be cruising in 5th gear and when you downshift to 4th or third and let up
on the gas, you can feel the engine bucking evrey few seconds…Throw it in neutral and idles fine never stalls. This is more annoying than anything else
and gets good MPG 20+ on the highway.

I set the base idle up according to the shop manual by disconnecting the IAC, setting the TB idle adjustment screw to correct RPM then reconnecting
the IAC. When the bucking happens the tach remains steady so not a loss of primary ignition, as if the Ignition module was bad or acting up you
would see the tach drop or flutter but remains constant.

Car will run right up to the redline as if you were on a drag strip with no hesitation hitting 90 mph in 3rd gear. I checked the codes and
with the jumper clip and nothing being stored and no check engine light when this happens.

This started about 3 years ago and know its not an EGR fault as ran fine for years after blocking the EGR.

Any Mustang guys out there have a suggestion ?