Hi Lee - welcome to the forum. Is that the ARC One American Classics set you have? And has that problem always been there? You haven’t modified anything or added any extra track pieces? Finally, you do have the two cars on separate lanes (of course you do!).
If the problem happens with the original set as it came it of the box, you most probably have a faulty powerbase. Contact the retailer where you bought the set to exchange it for a new one. Alternatively, contact Scalextric Customer Services (https://www.scalextric.com/uk-en/contact) for a replacement.
The only other reason that might happen is to have an odd number of racing crossovers in your track (not included in that set) - you should always use two, four, six or none.
Hope that helps. Great set, by the way. I love the Scalextric muscle cars :-)