Problem replacing audio in video

I have a video of my band at an outdoor gig where the audio track is fuzzy. The mics were overloaded somehow. I have, with a bit of effort, managed to get the audio and video (with the correct frame rate) into a Project, on separate tracks. My plan is to record new audio to match the video. All new tracks, both audio and instrument tracks, record fine as long as I don’t show the video track. HOWEVER, when I play the video track to make sure I’ve matched the new audio with what appears on screen, I get heavy distortion on all tracks after about a minute. Muting makes no difference and I can’t see where it’s coming from or how to get rid of it. So I have to bin the project. Can anyone tell me what I’m doing/not doing?
I’m using Cubase Elements 9 and Windows 7.


Try to increase Buffer Side, please.