More clues re. odd acceleration effect --

When using the accelerator in neutral, no problems at all. It works exactly as it should.
Checked many times.
The fault happens right on the gear change from 1st. to 2nd. I feel it change rather roughly, then no power until I depress the pedal a lot, or take my foot off and replace.
I tried giving the pedal a slight waggle (so as not to affect the vehicle's motion) just before the gearchange, and it stopped the fault. This action repeated many times with the same result.

Leedsman.