Slight miss when cruising

If it's happening under light throttle with heavy load/ 3/4 gear, it could also be too much advance. The engine is already fighting your rear tires under that condition, so if you've got more advance, like the old man says, you've got a situation where everything starts to try and slow the crank down on both ends, from the tires/ gear its in with leverage against crank and from the piston coming back on itself a little.

Back it off about two degrees and see what happens. If it still messes with you, start looking at fuel pressure and fuel levels in the carb.