Please help suggest a cause of a new problem in 1981 Westie (aircooled),
automatic not stick shift.

Occasionally, a few times a day, there's no response from the engine when
pushing the accelerator pedal to accelerate from stationary. The pedal
moves normally, and the sound of the engine changes so I think the throttle
opens, but there's no change from idle, no increase of power for at least
several seconds. Then the engine behaves totally normally. I've begun
pumping the pedal when this happens but I'm unsure if it makes any
difference.

The engine was rebuilt a couple of years back (lovely job by Greg Tuttle,
installed by Fred's Garage, runs like a dream) maybe 10,000 miles ago. It's
had oil changes but little else since then.

Any ideas how to identify the cause will be much appreciated. The problem
isn't reproducible on demand, it only happens now and then - and can be
scary if I'm pulling out across the road.

Thanks
Alan