Does it happen on say just the VCR menu screen or some other stable source too? IMO that looks more like some electronics issue than macrovision (which I thought the TBC-1000 shouldn't have an issue with in any case.). The dropouts look way too abrupt, and not spotting the white macrovision dots on the top/sides.

Second the suggestion about line tbc though, as long as you're not using a VCR with line TBC built in (and not dealing with macrovision), a dvd-recorder with line-tbc functionality like the ES10/15 will do a much nicer job than the TBC-1000 is capable of since it's not capable of doing a lot with horizontal jitter and flagging/tearing.

Since you as noted already have access to the Toshiba (and maybe the panasonic) you can see how it compares (though as noted the Toshiba may have some brightness flicker and the ES47 may not work, so looking for a ES10/ES15 may be preferable.)

I don't think this is Macrovision, or at least "real" Macrovision. The Three Stooges and Brazil tapes were recorded off of cable TV, and the Transformers one appears to have been dubbed from a rental tape (so I would assume if Macrovision was present that copy would have been unusable).

I also recapped the Transformers tape as a test, and there are no glitches in the second capture at this location.

I did run some tests previously with the Toshiba DVR620 and Panasonic DMR-EZ47V as passthrough devices. They both appear to help with the flagging, but the Panasonic introduces a dithering-like checkboard pattern in solid colors (at least). However (IIRC) at least one of them (but I don't remember which) also introduced some compression-artifact like distortions in the flagged area they fixed. My VCR is broken so I can't provide samples. My initial goal has been to get one full pass of acceptable (e.g. not too glitchy) captures of all my tapes before going back and trying more cleanup if that was necessary.

Would an ES10 be effective enough at fixing the video signal to avoid audio sync problems without a TBC-1000, even on old tapes? Also, I have the impression that the ES10 is "better" than the ES15 as a passthrough. Is that correct?

Yeah if it was macrovision it would always happen at the same spot since it would be caused by the video chip reacting to the white blocks and other macrovision stuff that is in the video signal on tape.

Checkerboard-like effect on colors can be some electronics issue like bad caps. It could also be luma/croma separation artifacts though since you are using composite output, there is a fair bit of that on the samples you posted, on those it looks like diagonal lines instead.

You can get most of the work done by acquiring digital copies on DVD or Blu-ray and capture the ones that didn't make it to the digital era. I know the 3 stooges did exist on optical discs.

That wouldn't actually fulfill the goals of my project, and even if it was I don't actually know exactly what's on a lot of these tapes. Many aren't labeled and the rest are ether labeled inaccurately, incompletely, or vaguely.

This isn't anti-copy, or a false positive detection. The unit has an internal error. I've seen this many times, and it's often something that I have to refurb for the DataVideos. This issue is very common with all those crappy "tested" and "working" units on eBay. Replacement parts are needed, either OEM or newly made. A complete bypass is also possible, but with much more work.

Do you have Transformers toy commercials on those tapes!? (Or are those retail?)

Not on that particular tape, which turned out to be (I think) a dub of a rental video (it does have some promos for GI Joe and Thunderbirds In Outer Space). I do have another Transformers tape (which I've been saving for last because the beginning was long-ago eaten) that might have some. It definitely has some GI Joe toy commercials, though (the Cobra evil twin brothers, which I remember well, and one that has the jingle "GI Joe Sky Striker flying high, going to knock the Rattler out of the sky").