first thing i would do is put an amp probe around L1 or L2 either at the service panel or in the dryer near the terminal block. dryer should pull about 22 or 23 amps.. you can check cont. of element to ground and also do a visual.. depending on make of dryer...if its ge or fridg. the elements are behind the drum and not easy to inspect.. others are behind toe panel or back panel and are relatively easy to access...
i have seen loose connections at breaker cause circuit to trip.. could be the drive motor siezing up. put amp probe on motor to see if it excessive..
if amp draw is less than 25 at pigtail and it trips , you still have a defective breaker or perhaps a loose connection.
if over 30 amps.. use amp probe to isolate if element is drawing too much or motor.
the fact that it take 5 minutes to trip tells me the breaker or connection is faulty or motor is siezing up as it runs but the motor should typically cut out on the thermal overload...???
I hope this helps