at intervals
Separated by a particular amount of space or time. Make sure to position those Christmas lights so that the flashing ones appear at intervals.
at (...) ˈintervals
1 with time between: Buses to the city leave at regular intervals.The runners started at 5-minute intervals.
2 with spaces between: Flaming torches were positioned at intervals along the terrace. at intervals
In a series separated by space or time: trees planted at intervals; coughing at intervals.