Stuttering Analogue cars

Open circuit voltage readings taken on a track with poor connections are actually almost meaningless. The reason for that is that poor connections act like resistors and the voltage drop across a resistor is proportional to the current flowing through the circuit. The method that you used would only find a problem if something was completely open and even then there would have to be several opens on the same rail. With Sport track the rails can be a little lower than the track surface, so you may need to fiddle with the car's braids to make sure that they are not just riding on the track surface. In order to troubleshoot the track you need to disconnect the last track section from the section with the powerbase. Put a car on that last section with sometrhing under the back of the chassis to get the rear wheels off of the track. Strap down the trigger of the controller for that lane, hopefully the motor will run, if not you will have to move the car backwards around the track until it does. Now you can start at the connected end of the track and locate the bad joints. You can take voltage readings across the joints on the same rail, if the joint is bad you will read the voltage drop, if the joint is good there will be no reading. Rail to rail readings will find the section that has lower voltage, but it won't tell you which joint is defective.

If you leave the track sections connected all the way around then power can get to the car from two directions, making it more difficult to find bad connections. Connections that are completly open are easier to find, but for a car to actually stop running there has to be a break on both sides of the part of the track that is giving you trouble. With the track disconnected at one end you eliminate the confusion of having two paths for the electricity to take. If there are indeed some poor connections the problem would be worse with cars that use more power. More powerful cars use more current, the more current the bigger the voltage drops across the bad joints will be and the voltage that the car sees will be lower.
I am still not certain that the braids are making good contact. If the braids get splayed out they can ride on the plastic and not make good contact with the rails. It is helpful to put a little twist in the braids so that they make a shallow V.