A comeNgo issue is very nearly always in the fuel system.
But to save chasing my tail, I always check the ignition first. If you have flyweights in the dizzy, check them for smooth,non-sticking, snap back to park, operation. This is easiest to see with the timing lite.
1)The best time to check the fuel system is right when its acting up. The very first thing I would check for, is,water in the gas.It likes to accumulate in the bottom of the float bowl. When it gets high enough,it goes through the jet or jets, and causes anything from a momentary nose-dive while driving, to a stall-out at the stoplight, to intermittent rough-idling, to a nose-over during acceleration.
2)If the fuel is good, I would blow out the low speed circuits.
3)And lastly I would check the fuel level and prove it stable.
I think you will find the problem in one of these three.
FYI; a vacuum leak is usually a full time issue, as opposed to comes n goes. A sticking advance weight(s) usually just causes a high idle. A broken advance spring usually causes driveability issues when the advance comes in way to early.