Hi Pete,
It sounds like the problem you are seeing is down to the quality of the output of the VHS output. VHS, and certain other tape systems produce very unstable composite outputs. TVs and monitors do not typically mind this, but the Intensity Pro needs steady sync for it to capture correctly. A Timebase corrector (TBC) may help, but depending on the condition of the tape it might not always be perfect. Some VHS players may also have a TBC built in. I don't have any recommendations for a TBC, but others who have used them may have some suggestions.