Iris+ and copter 3.5.1 - Inconsistant Heading

Any help/advice on this would be much appreciated.
I have and Iris+ for almost two years now and I use in for mapping flights so in spends most og it’s time flying auto. Up to press it has been first rate and does exactly what it should. Started with copter 3.2 , 3.3.3 - 3.4 and in August 2017 went to 3.5.1 - all looks very stable and works well. Did a couple of commercial mapping jobs and I stop unil I started agin if Feb 18.
After a bit of free flight I did a test auto flight which went well except the the Iris did noy follow the way point heading but just stayed in the orination it was in after take off. I checked my flight plan and replayed the flight log - nothing there.
3.5.5 had come out so I upgrade and re-calibrated. test flew - free flight - OK good - auto flight - would not follow waypoint heading - still the same issue. Found the WP_Yaw_Behavior and it was set to 2, which was the behaviour I expected. Changed the value to 3 which facing the gps and the Iris then behaved in the correct manner following the heading. Couple of test lights - behaves OK, consistant with the setting. I had a mapping flight to do composed of two auto missions so did the first flight - flew great. Then on the next one it reverted back to not doing anything? Just about to complete the mission when I get a velocity varience error, it shot to the right and we had a crash landing. Logs show a gps glitch but I’ve not had one of those in two years flying.