My '74tii has been known to do that. My fixit has been to tweak the tiny screw inside the tuna can.
For example:
a) I would do a tune-up: clean plugs and points, set point gap. Then the up-down idle cycling would happen. So with the engine "on" and at normal running temperature, I would turn the tiny screw 1/4-turn (I don't recall the direction) ...... idle would smooth right out, everything was just fine.
b) 1,000 miles later, the idle would start cycling again. So I'd turn the tiny screw 1/4-turn in the other direction ....... idle would smooth right out.
c) Then I'd do a tune-up ...... and again have to tweak the tiny screw.
d) The "secret" I discovered is that the tiny screw need only be turned no more than 1/4-turn, maybe even 1/8th of a turn, to provide the steady idle.
e) In general, I didn't need to adjust the idle speed screw on the tuna can.
Cheers,
Carl