It sounds like your locos are stalling on the switch frogs. They are the Y shaped sections of the switch are that are usually made of plastic. What happens is that the spacing of the electrical pick-up wheels of your locos are encountering these electrically ‘dead’ sections of track and stopping because they have no power. The fact that slightly increasing your loco speed solves the problem indicates that your engine has enough momentum to ‘coast’ over these dead sections.

I’m not sure what to suggest for a fix though you may want to try these tips and hope they work. Sometimes, during the molding process of the switch, the black plastic will coat a little more of the metal rails than is intended in the frog area. Try to carefully scrape the top of the metal rails with an X-Acto knife TOWARDS the frog and see if you can reveal any metal track beneath. This coating should be extremely thin so be careful not to gouge part of the switch that is all plastic. You may only reveal an 1/8 or 1/16 of an inch of metal track but this may be enough to solve your problem. If you prefer not to scrape the rail head as I described, get a precise measurement of the exposed metal rail on the switches that DO work and compare it to the switches that give you trouble. If the measurements differ from the ones that work compared to the ones that don’t, the rails will have to be scraped down as I discussed to solve the problem.
One other thing that might be the cause of your trouble is that the switches may be nailed down too snugly to the road bed or table surface. If this is the case, the switch track is actually distorted or slightly twisted which causes the loco’s pick-up wheels to bridge sections of powered track, resulting in the stalling you described. Increasing the speed solves this problem for the same reason I mentioned above. The solution (I think) is to raise or remove the nails that are holding the switches down too tightly. As a rule, I DON’T nail down my switches at all and allow them to ‘float’ between sections of my track that are secured.

Hope this helps,

Geof