Guided mode YAW keep Changing its orientation


i am running AC 4.2.1 (Hexacopter) on CUAV V5+ flight controller with dual GPS configuration.
its flying well in all modes like stabilize ,AltHold ,Loiter and Auto but i have noticed only one problem that when its flying in Guided mode at some heading its not holding that heading .

YAW is keep changing at 45 degree or 90 degree at random time intervals .
its not compass issue because I have seen no error message in mission planner message tab when YAW changed automatically.

this is not happening in any other modes except GUIDED . Auto mode YAW behavior also fine no yaw issue like in GUIDED mode I’m little clueless please help.
here is the log file Guided YAW issue - Google Drive