1987 Cougar V6 misses after warm up

I have 1987 Cougar 130,000 miles that runs good until about 10 to 15 minutes of driving it will start to midly miss, you can feel it shudder when driving. It does not miss under acceralation just on
crusise. It seems worse around 45 when climbing a small hill, but you can stomp on it and not feel it miss. At crusise around 70 to 75 there is this small shudder. When it happens it is intantanous, but it is just after driving 10 miles or so at highway speeds. I have given a complete tune up a year ago, plugs wires,cap, rotor air filter,PVC valve. No engine check light comes on. Any ideas?
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No hesitation or idle problems, otherwise?
What brand plug are you using?
I suggest you have the codes read out... sometimes a DTC code will exist without setting the light
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Are you sure it's not a torque converter shudder?
CJB
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If it has a distributor try replacing the rotor and cap. Could also be the start of a problem with the TFI (thin film ignition) module bolted to the side of the distributor.
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