English Translation
From time to time she stopped and looked round.
“From time to time she stopped and looked round.”
I’m thinking that the preterite (paró, miró), would perhaps feel more natural, unless this was a habit.
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English Translation
From time to time she stopped and looked round.
I’m still wondering. 
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Paró y miró would be a one time thing.
Cada cierto tiempo
Indicates a recurrence, as you said a habit, the imperfect tense is correct.
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Even for a short-term habit? If you pardon a more drastic setting:
“Cada cierto tiempo los terroristas sacaban / sacaron a un rehén y lo mataban / mataron” ?
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From what I know, we use Pretérito Imperfecto for ongoing, habitual or repetitive actions in the past, without indicating a specific moment of beginning or end. And Pretérito Perfecto Simple for completed actions that happened in a specific moment in the past, with a defined beginning and end. (From that I say it’s imperfect but I’ve been repeating the imperfect and preterite all over again in my head and I’m getting confused now 
)
And this I’m not 100% sure but it sounds more natural to me the imperfect because Cada cierto tiempo indicates a recurrence. Maybe waiting for someone else to answer is not a bad idea.
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