I experience the non-responsive throttle pedal almost always in first gear when trying to pull away. Engine gives very little power and if I give it progressively more throttle, nothing happens until all of a sudden car takes off rapidly often spinning the wheels. Makes driving in slow moving traffic very difficult and almost dangerous.

As I said, happens in other gears too after coasting with clutch disengaged or lifting foot off throttle pedal to slow down.

The surge in revs is when changing gear, so foot is already off the throttle pedal, this is much more intermittent than the loss of power.

I have not experienced any connection between the surge in revs when changing gear being preceded by lack of power.