Possible Clues:

• A tobogganist turning up on track — as he must train?
• Man in services seen with empty lorry on a frequent basis
• Extremely unfamiliar allergy treated at equal intervals
• Criminal goes missing from rogues' gallery surprisingly often
• ls familiar to barman in the middle of analysis often
• How shoes will appear in schooners, at intervals
• Evenly
• At equally spaced intervals
• Often — at intervals
• At fixed intervals