Did you use the composite input or the s-video input on the Grabby, and what is the video source and which outputs did you use?
This problem could be caused by using the S-Video port of the Grabby and connect it through an adapter(e.g. SCART) to an external device which is not equiped with a S-Video port. If using a SCART plug on the video source do not use the s-video output - see image
Come back with more info then we may be able to help