Actually since 160 is not a standard OCPD size you are permitted to use a 175 amp breaker. However, you must protect the secondary conductors at or below their rated ampacity, so changing the secondary breaker may also require you to change the secondary conductors.
As far as why it is tripping there could be some type of intermittent load that is high enough to cause the breaker to trip. Maybe an intermittent fault. Or if could be a bad breaker.
