Erratic throttle response when cold

[HPsauce]

OK, so here's a really INTERESTING bit of diagnostic data.......

The catalyst was replaced yesterday and not long after I "thought" I detected a hint of the problem, but in a few short journeys since it's been OK.

Then an hour or so ago I had the opportunity to take a non-optimum route somewhere but one that often creates the symptoms. And it did! So, not the catalyst as a cause.

Now, it always behaves fine on cruise control, so I had wondered - what happens if I switch cruise on while it's having a "go-slow".

The symptoms appeared as I turned slowly into a main road with a 30mph limit, so I let the car accelerate gently (throttle pedal pressed some way down to do that) then hit the "set" button at 30mph without moving my right foot. I don't recall exactly but it was probably near 3000rpm when I did that - would that be 2nd gear?.
Wallop! It instantly set off like the proverbial scalded act!
Took my foot off and it eventually slowed and settled back down to 30mph on cruise.

So, what does that tell us?
I don't think it took the time to do anything in the gearbox, the response was literally instant, going from gentle to heavy throttle, as if it had reread or reinterpreted the pedal position.

Any and all thoughts welcome......

I'll try it again whenever it happens and it's safe to do so. Anything worth noting at the time or changing?
(one thing I might try next time is pressing the cruise set button at a lower speed, below the normal activation limit which I think is 25mph.)