Power?

You may have poor connections between some of the track sections, that will give you a lower voltage in some places. Unless the sections with poor connections are the ones where jumpers happen to be connected jumpers will not cure your problem. Before you do anything else take a good look at your track. With Sport track sometimes the rails can be a little lower than the track surface here and there, if that is the case if the cars's pickups are not adjusted properly your cars will slow down on those spots because the braid will ride on the track, not on the rails. Twisting the braids to make a shallow V when viewed from the back will give you better contact. If tinkering with the braids does not fix your problem you should find and repair any bad joints before you consider adding jumpers or copper taping your track. Finding the bad connections is complicated by the fact that power goes both ways around the track, so in order to troubleshoot the joints you have to dissconnect the track on one side of the place that power comes in. If you then put a car on the side that is still connected you can drive it slowly until you find a place where it wants to stall. When you have located a bad joint you will have to open it up and bend the contacts to get a good connection before you continue around the track. There are more sophisticated ways of troubleshooting a track that include putting a load on the circuit and taking readings with a volt meter.