There are power breaks or weak power sections on the Scalextric layout. Check here step by step how to repair.
When a car stops on a circuit this can indicate TWO breaks in electrical continuity since electricity travels in both directions around the track. If there was only one break then you would not notice it as electricity flows in both directions!The way to correct this problem is to:
Disconnect the track behind the power base.
Then drive a car forward at a slow speed. When the car crosses from one piece of track to the next there should not be a drop in power/speed.
If there is a drop or loss in power, then fix the track connection. Disconnect both track pieces (the track piece immediately before and after the fault). Turn them over on a hard surface. Ensure that the metal rail tabs at the ends of the rails (there are 8 of them on every track piece) are fully pressed and clamped down to the plastic. Use a stout screwdriver or similar to push them tight
Reconnect the repaired track pieces and retest by placing the car back at the powerbase and again drive the car slowly forward until you meet the next reduced or lost power section.
Continue the above repair process until you get to the last track piece before the powerbase.
Reconnect the last track piece to the powerbase.
Full power should now be restored.