is your field reasonably flat/ level?     My similar  zimm issues  were on a field that  had several hills and  a  big  draw that flexed   each joint  quite aggressively.  But this is on a 20 YO  Gen II   system..   the swivel joints on the control rods  gotten pretty crusty and   an angle change up or down  would  safety it off.. even though it appears  perfectly in-line.
just hitting restart  usually  jiggled it enough to  be fine until the next  round.    
I'd suggest next time it  does this,  but before you  get it running....  go down each tower to visually see  which  safety switch is tripped.  You may only have a  bent  micro switch arm that  puts it on the very edge of  safety-off,  even though the  'run switch'  is keeping it perfectly in-line.     

any chance you could run it the opposite way to see if it  still happens?