oscillate
Definitions
To move or swing back and forth at a regular speed.
To waver between extremes of opinion, action, or quality.
To vary in magnitude or position in a regular, periodic fashion.
Examples
The needle of the compass began to oscillate wildly as we approached the magnetic pole.
Her moods seem to oscillate between extreme joy and deep melancholy without any apparent reason.
The pendulum will continue to oscillate back and forth until friction eventually brings it to a stop.