1972 Oldsmobile Cutlass Surging/Loping/ No Power - came and

I drive the car about 2 - 3,000 miles per year.
The engine has about 100k on it, and the carb was bought rebuilt/new (I forget) about 12 years ago.
Well maintained. Always performs smoothly.

I drove the car about 21 miles, 16 of that highway at 70 +/- mph. Outside air temperature about 65 and dry. It ran smooth and strong as usual.

I went to pick someone up. The car was shut off about 5 minutes. When I restarted and drove off, I noticed sluggish response of gas pedal, though the car would idle fine and not stall when I came to a light. (I should note that on hot restarts, I sometimes have to crank it a while, but I attribute that to the gas?)

When I tried to accelerate, the car would lope/surge, as in seemed to move ahead, then move ahead slower, and had weak power. I returned using back roads because I didn't think it could make it on the highway. To accelerate away from a light, I would have to push the gas pedal about a third the way down, or else it would lope. If I wanted to get to 30 mph, I would need to gradually push the pedal to the floor, and even then it wouldn't shift into third gear until it got up to about 40 mph, and I backed off the gas pedal. To go up a hill, I needed to almost keep the gas pedal to the floor.

I pulled in my driveway to see if anything was loose or out of place. I never shut the engine off. I opened the hood, took off the air cleaner, and pushed the throttle a few times. The throttle responded normally, no surging, weakness or unevenness. I thought maybe a vacuum line to the transmission was cracked or off, but no.

I then thought I'd check it again, since the throttle responded well.

When I left the driveway, the car worked fine as it always had, and I drove it about 12 miles without incident.

The question is, what happened to make it lope/surge, and why did it go away, and will it come back?

Thanks