I had a similar question on this forum (why doesn't my fan work whenever the A/C is on?) just a few days ago, and found out that this intermittant operation is actually normal. It's controlled by a pressure sensor, and it only reaches this pressure occasionally and, after the fan adds its cooling contribution, isn't needed anymore. In fact, it was pointed out that often the fan won't go on at all -- most often it only works during periods of idling (when the regular engine fan isn't pulling enough air through the coils) on particularly hot days, rather than when the car is being driven.
In my case, since I have a tropical fan clutch, it didn't seem like this electric fan ever runs.
Bottom line, it's normal.