I suspect a weak or loose control relay in your A/C unit interface board, probably the fan relay. You don't specifically say, but is the compressor continuing to run while only fan cycles on and off every 5 seconds?
I might also be inclined to rule out the thermostat by jumpering Rc (R) to the fan (G) and compressor (Y) terminals at the thermostat sub base, effectively bypassing the thermostat. If that stops the cycling, the thermostat is the problem.
Normally the defrost switch will only cycle the compressor and leave the fan running. Low airflow or dirty filter would cause freeze up of the coil after running for a while, not 5 seconds. A weak start capacitor will prevent the compressor and/or fan from starting from an off condition.