Sounds like you've tried the obvious things and it is something else. Here's some ideas about what the obscure fault could be.
If the problem is only happening in particular parts of the track, its quite unlikely to be motors on the way out. If a motor was defective, it would be expected to be defective all the way round the track.
Are the cars driven with the tail out where you have the problem? Just possible the conductors in the lead wires are cracked inside the insulation and this only shows up when the guide is turned.
Are there changes of gradient or banking where the cars are stuttering? This can cause pick up problems.
When you say you have tested the conductivity with a meter, exactly what have you measured?
Simply measuring the voltage with no load applied doesn't detect poor conductivity at track joints.
Measuring the voltage when the track is passing a current will show poor conductivity at track joints.
If the problem is only happening in particular parts of the track, its quite unlikely to be motors on the way out. If a motor was defective, it would be expected to be defective all the way round the track.
Are the cars driven with the tail out where you have the problem? Just possible the conductors in the lead wires are cracked inside the insulation and this only shows up when the guide is turned.
Are there changes of gradient or banking where the cars are stuttering? This can cause pick up problems.
When you say you have tested the conductivity with a meter, exactly what have you measured?
Simply measuring the voltage with no load applied doesn't detect poor conductivity at track joints.
Measuring the voltage when the track is passing a current will show poor conductivity at track joints.
