On reboot my ESP sat in a loop at 78800 reporting the following:
Fatal exception (0):
epc1=0x40100003, epc2=0x00000000, epc3=0x00000000, excvaddr=0x00000000, depc=0x00000000
I finally discovered it was because I had GPIO2 pulled low during boot (I had a switch connected to the GPIO).
It appears GPIO2 must be high or this exception occurs.
Hope this helps someone.