Movies over an hour restart and goes black

hi, have got 6 pi's in sync works great but the movies are 1h 3mins 43s
after about 1 hour the screen goes black screen every time for up to 2 seconds then the others restart and pause to re-sync with the master. 

am using EXFAT USBs with the video on them, and a USB sound card just on the mater
all videos exported as the same codec, 
all wired into a network hub. 

any ideas anyone?
