If the breaker is tripping it could also be a faulty
heating element. As it heats up the hot wire inside expands at a faulty spot until it shorts out to the cover, hence the time delay involved.
With the breaker off, measure the resistance through the
heating element and
record it. After the breaker has tripped and before it cools, measure the resistance again. It should be higher and zero or close to zero would indicate a faulty element.