[StuBeeDoo]

So do they stop in different places around the track, I.e. not a track conductivity issue?

Do they stop if you wiggle the power lead on the DS? (Broken cable/solder joint?)

Do they stop if you put side pressure on the trigger? (Wiper arm lifting off wiper board?)

NOTE...I'm no electrician!

Thank you for asking.     Those are points I should have cleared-up.

No, it doesn't appear to be track conductivity, because the points where the cars stop are random.
Wiggling the cable makes no difference.
Pressing the wiper arm onto the board makes no difference.

..........And I'm no electrician, either.