Random images from a session not recognised in Lightroom-shooting tethered from a canon 5Ds

Shooting tethered in LR, it randomly rejects images and says it cannot read them, the frequency goes up and down but its too frequent to ignore ranging from 1 to 4 failed images every 10 frames. 

the tethered lead is new 'Tethered tools'.

You get a grey thumbnail with an exclamation point saying LR can't read the file and you cant make adjustments. during a 1 day shoot I lost 50+ images to this issue. Also LR crashed 4 times. 

Has anyone else exerienced it or solved it?