You also might take a look at your spark plug wires. They could be just out of spec enough that the prolonged heat from long drives makes them perform badly. I know my carbie bucked like someone was dancing on the brakes when the original wires finally died after 17 years.

OEM, NGK, or Denso are the best choices if you do determine them to be the problem.

My Truck: 1987 XtraCab DLX 22R 4WD 5 Speed Manual
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"Speed has never killed anyone. Suddenly becoming stationary, that's what gets you." -Jeremy Clarkson