This is a very common occurrence with the 36-553 & 36-554 decoders. CVs 54 & 55 are the ones to play with. CV 54 to correct starting performance and CV 55 stopping. There is a list of suggested values for a selection of locos on the Bachmann website. http://www.Bachmann.co.uk/pdfs/decoder_settings.pdf
Which terminal is marked as NC (normally closed)? That's the one that will be feeding the frog because the switch is in it's normal position IE unoperated.
I had a voltage loss between two ordinary pieces of curved set track. Temporary jumper wires proved it was the track joiners at fault and two more connections to the bus provided a permanent fix. This was pretty new track which hadn't been ballasted. ( Still not ballasted BTW)
Soldering track together is a bad idea if it is subject to temperature swings.
Rather than use short loops across the rail joiners I would simply solder wires on to each rail from a power bus.
With loops across the joiners you are relying on the rails to carry the current and the rails have more resistance than a power bus and short wires from a bus (droppers)
For reliability, every piece of track should have a dropper. It's up to you to decide if you want to follow this "rule". Some people only have one pair of track feeds for the whole layout and say they don't have any problems. That's their choice.
Looking at the pdf instructions, it isn't a stall motor. It has limit switches to cut power at the end of stroke, so no power is taken when not moving.
You want more than 12volts for the NCE Powercab or the locos will probably run rather slowly.
There have been many reports of slow running with the Powercab when a lower voltage power supply is used. I can't remember for sure but I think the maximum is 15Volts.