1982 F150 straight six 300ci motor cutting in and out on highway...

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I have a 1982 f150 2 wheel drive with the straight six 300ci 114k original miles...The truck drives fine in town, but on the highway when you get up to 65mph or so it will start cutting out....It does not have this problem when in neutral....only under load...any ideas?? truck has Cat, no muffler, I have replaced all of the following within the last month plugs,wires,fuel sending unit,water pump, alternator,thermostat,radiator. Any help with this would be great...