64 valiant 225

running warm, but cools down at highway speed at night.

just changed thermostat and coolant.

runs little thumpy at idle and has stutter when first accelerating.

san diego to el paso today, barely made it out of the desert then ran fantastically smooth at night. should i be making some adjustments to carb or elsewhere?

oil good, trans fluid good, cooling system flowing. i have a slighlty heavier than usual load (i.e. a passenger and bags instead of just me) but not towing a trailer or anything and hadn't had any issues prior.

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64 Valiant 225, 4dr

Low speed overheating is usually linked to poor air flow thru the radiator.
Is tha fan with-in 1 inch of the radiator?
Is the front side of the radiator clean? (low "dead bug count")
DD