First step is to find out whether it is caused by the camera or if it is on the VMS side.
You could ask the following questions.
1) Next time it happens, log on to the camera via the camera web interface at the same resolution and fps as the stream the VMS uses to connect to that camera if possible, and see if the problem exists on the live page of the web interface. If so it is most likely a camera problem, if not, it still might be caused by the camera or the VMS side.
2) After you rebooted the computer and the problem went away, did you try playing back video that was recorded when the problem existed? If the video plays back correctly, then that tends to point towards the VMS side.
3) Does the stream from a particular camera always display "split" on all client workstations displaying that camera, or does the same camera play broken on some but no others? If the same camera displays OK on some, but not others, then it also tends to suggest the VMS side.
4) There are multiple computers running the client UI where this problem happens, are these all the same model of computer? more importantly are they the same video card?
5) How many camera streams are being displayed simultaneously on the UI e.g. 4 by 4 tiles, 5 by 5...?
6) What video card are they using?