every other (?.)

• alternate
• alternately
• beating
• by turns
• circling
• cyclic
• epochal
• even
• in rotation
• in turns
• intermittent
• isochronal
• make and break
• measured
• metronomic
• off and on
• oscillatory
• periodical
• pulsing
• reciprocal
• reciprocally
• recurrent
• recurring
• rhythmic
• rotary
• round and round
• seasonal
• serial
• steady
• to and fro
• turn about
• undulant
• undulatory
• up and down
• wavelike
• wheeling