1989 Honda Prelude Tail lights

Was driving behind husband's car at night. Noticed that the tail lights were pulsing at regular intervals. Pulsing for 10-15 seconds, then normal, then pulsing again. Car not going more than 40mph. Did not appear to do it when stopped. Brake lights seem to work normally when stopped - all three light up. Almost looked like emergency flashers, but only red lights. Head lights are unaffected. Also, no check engine or warning lights. any ideas?