GPRSbee V7 DTR crashing problem

Hi, I’ve been using the GPRSbee for some time, V1.2 and V6b - I’m now putting together a new piece of hardware which is exactly the same as previous versions with the same code but using the latest V7b GPRSbee. The problem I’m having is that when I enable DTR to turn it on, it’s crashing the MCU. I have 2 of the V7b units and they are both the same. I’m not sure quite how to diagnose the issue - if I drop a V6 GPRSbee in there is no problem so definitely related to V7. Has anyone else had this or can suggest a route of enquiry? Cheers.