I have am NCE Power Cab. It has plenty of "juice" (over 1 Amp) to power any N scale model with a sound decoder. Power Cab also has a setting where it shows the current going to the track on the display. It will show much less than 1 Amp.
Seems to me that it is a problem with electric pickup. When the model moves slowly, it will be very susceptible to loss of connectivity with the track and will stall. Also the decoder's rather small built-in keep-alive caps will not keep it powered through the dropouts. Running faster the flywheels and the inertia of the moving parts will let the model coast over the power interruptions (which are much shorter at speed), and the decoder's small internal keep-alive caps will keep it powered. Sound decoders are sensitive to that. I would make sure that the loco wheels and the track are absolutely clean. That is why many sound decoder installers add extra keep-alive caps, or even a full keep-alive module. Regardless, clean track and clean wheel treads are vital to smooth operation, but it is extra important with sound decoders.
If you can, hardwire (using test clips) the loco to the Power Cab track output and run it sitting on the workbench. See how it behaves. If it behaves well at all speeds, then it confirms that the electric pickup is a problem.