There was some correspondence on the nemetode.io group (which I heartily recommend you join) about peculiar captures…
Are you using Windows 10 by any chance…?
If so, the conclusion, after exhaustive tests just like you describe, seemed to be to change to win 7 or even XP. I have systems using both these operating systems for over 10 years and they’ve never hiccuped. I’m not an expert and I don’t know why but Windows 10 does seem to have some issues.
That may be the main issue but I could also add, from experience, stick to onboard hard drives, using external ones seems to cause problems with recording. Mine was/is a western Digital USB3 model and it just didn’t seem happy…
Not much of a solution but I hope it helps.