In addition to the suggestions abouit the stability of the original VHS recordings, it might be worth considering the behaviour and features of the capture device too...
- When capturing directly as MP4, does it do that "on the fly" as it captures, or does it capture in some intermediate format first and then save as MP4 once the capture is stopped?
- If capturing directly as MP4, how much CPU usage is there? If CPU usage is high (say an average greater than 50%), does the device offer other capture formats? If it provides it, I would suggest DV-AVI (this is quite often done by hardware inside the capture device itself, so much lower CPU load). Also DV-AVI uses "intraframe" compression only, so is much more tolerant of frame drops/frame skips than something that depends upon groups of frames (MPEG-2, MPEG-4, H.264 etc). File size will be larger, but you can always convert to MPEG-2, MPEG-4, H.264 etc. for archiving. And, editing DV-AVI is much smoother than editing in a highly-compressed format as there is much less CPU workload while previewing, etc.
- Does the capture device have a setting to improve the capture of poor quality tapes (some Pinnacle capture devices had a "VCR" setting that made them more tolerant of less-than-ideal video signals (enabling you to capture something rather than just considering the unstable frame(s) as not being valid and throwing them away)!
HTH
Richard
Added: I Googled for the video grabber's spec - apparantly doesn't offer DV-AVI, but it might be worth trying the capture as an MPEG-2 file as that codec might be less CPU-intensive than H.264. Another idea may be to try it in a different USB port in case it's conflicting with some other device