Sound Troubles with Mini HD

I have been using Blackmagic Mini HD for a few years and have captured about 1000 hour of DVCam and mini DV successfully. With this same set up, cables etc, I started digitizing VHS and Hi 8 tapes. Many of these tapes give audio problems once the tape gets to about 15 minutes and it gets progressively worse as the tape runs. When I was capturing with Quicktime, the audio would drop out intermittently. When I switched to BM Media Express there is a low frequency digital flutter. The tapes can be captured successfully if I capture segments of 15 minutes or less. So the tapes are fine. I use the same cables as I do with the DV and Mini DV tapes, so the cables are fine - plus I have switched them out. I have tried two different Mac computers - a Mac Pro and an IMac - with the same results. So it is not the computer. The only thing it could be, based on this process of elimination, is the Blackmagic Mini HD. I would appreciate assistance.

How do you capture the VHS and Hi8 tapes ? big chance you have sync issues….(you need a passthrough device)

For all captures you are using the Y BNC ? > Thunderbolt > Mac ?

You have LCD on (MiniHD) HDMI ? with okay picture and sound ?