Audio crackling with DVCAM and HD-Connect MI

So I'm trying to digitize a 480i DVCAM tape from a Sony HVR-M25U connected to an HD-Connect MI, upconverting to 1080i through a Kona 3 card in a Mac Pro. The problem is, about 30 seconds to a minute into capturing, I'll get crackling and popping noises in the embedded audio. This doesn't happen when I capture HDV tapes through the same setup. I've updated the firmware on the MI to the latest version--1.1.37--to no avail.

I suspect the problem might be a reference sync issue, but there's no Ref In on the M25U, so I can't feed the deck and the Kona 3 the same blackburst signal. Again, HDV tapes sound fine through the MI, so maybe it's not a sync problem. Any ideas what might be causing the audio crackling? I'm stumped.

Thanks!

Vince