It sounds to me as though you have a bad camshaft gear. Thats the likely culprit. The distributor shaft or drive gear could be bad, but, the cam gear most likely is missing a few teeth.
Possibly, but then it wouldn't be running great at 55 mph........
I suggest you look at all the associated parts. Pull the side timing gear cover (2 bolts on side and oil pan bolt) to inspect the timing gear teeth. Rotate engine full revolution to inspect the timing gear.
you can also check for backlash at that point ( although the cam is under load) but can carefully pry to see if you have excessive backlash. I doubt it though.
Correcting instructions....As indicated by another post about removing the drive gear assembly you do not "pry up on the drive gear against the spring"..... you remove the spring following the distributor removal and intermediate shaft, by pushing down on the spring and pivoting top of the spring out towards you... then lift the distributor drive assembly out of engine block. Inspect & service if tang/gear worn. Reverse to install.
It is normal for there to have a little play of the rotor rotation ( but not fit on cam - must be snug on cam). You can install a new intermediate shaft and check distributor mating slot for wear/fit .
I suspect if you have a lightened flywheel, or modified cam, etc.. that is your source of an idle not silk smooth, of course taking into account your carb and distributor are set/properly functioning. I would also check for a planed surface on your intake/exhaust manifold and good gasket as if they are sucking air they will cause poor idling ( among other things) as well.
I've tightening many a intake manifold bolts when idle didn't sound good. If you get lucky and the intake is flat and the gasket is still decent, just a little loose it's worth a check. I torque all evenly to 35 ft lbs.
And... I hate to make this simple but make sure you don't have any vacuum wiper leaks, sucking air! If in doubt put in an 1/8 pipe plug temporarily in the intake to eliminate it till you are for sure.
Setting points and timing is really simple on these... you don't need any fancy nurex tool, timing light, etc. Set you points according to the service bulletins taking into account the backlash and they will be right on every time.
Good luck!
Larry Shepard