To increase the overcurrent protection sensitivity, you can set the OCPTH register to 00 (it defaults to 01, and there are only 4 possible settings). However, overcurrent protection should not be the primary mechanism for limiting the current that the driver delivers. You will need to properly configure the current limit each time the driver is powered up (otherwise it defaults to the maximum programmable value of 18A per phase). How did you set the current limit (e.g. using our Arduino library or setting the corresponding register values individually) and what are you setting it to? Also, could you tell me more about your setup such as what stepper motor and power supply you are using?
Brandon