It depends on what you want to say and the frequency of the action: Periodically: from time to time; occasionally and regular repetition. Intermittently: at irregular intervals; not continuously or steadily.

The two adverbs behave the same grammatically. However, the meanings differ. You could almost say they are opposites. "Periodic" refers to regular repetition. For example, the motion of the earth around the sun is periodic. The seasons are periodic. "intermittent" describes a more random, unpredictable, repetition. Shooting stars are intermittent. Hurricanes are intermittent.

Grammatically there may be disagreement about periodically and intermittently being the same. Periodically suggests a regular period. Intermittently implies an irregular occasional unpredictable event or a repeating unpredictable period.

They're interchangeable. You could also say "occasionally." And to sound less formal, you could say "sometimes."