An intermittent electric burner is a simple fix and is similar to dead burned diagnosis. The customer will complain that the burner will not stay hot or get as hot as it used to. The connection within the socket gets hot during use and fails to make a reliable circuit. It simply means that the male end of the burner prong is not being held tightly by the female socket spring. Both need to be replaced. Cleaning or filing the male end is not a good fix.
Socket Kit
On rare occasion an infinite switch may be intermittent but that will usually become evident by wiggling the shaft or a clue from the customer.