Thanks for the info. I'd been struggling with this on the first loco that I've ever converted - on which I used a LaisDCC decoder.
As you said, it functioned OK at low speeds but as the throttle was edged open, it would suddenly take off like a scalded cat. (Am I still allowed to use that metaphor? Sorry if it offends anyone.)
I tried setting CV 61 to zero but that didn't help. However, subtracting 16 from CV 29 to suppress use of the speed table seems to have done the job.
I'd bought a few of these decoders as a batch, so I will now try out the others.If they show similar issues, I will move up market.
John