Don,
You have to make sure that the curved track section, and all of the screw holes in the switcher, have screws in them that are really screwed down tight. I had a similar problem, and found that since everything was not screwed down tight, the track and switch had occasional "high spots" in the rails, which caused the opposite side of the loco to lift up off of a rail, which can cause it for a split second to lose power. This can also happen if one of the rails in track section got slightly twisted out of shape, due to someone having to really pull hard or twist to yank an old pin out of the end.
I would start by throwing away the track sections where the problem occurs, and install new or nearly new track there. Then start screwing things down. Also make sure you install brand new pins and crimp them down hard with a proper track crimping plier.
Maybe this info will help.
Mannyrock