Fixing a jittery PAL tape source

[DominoDeer]

I'm not sure what exactly to call this, but I'm trying to clean up some old Thomas the Tank Engine episode copies.

The copies I'm using are the original tape masters from an older DVD set, PAL 25fps. The original interlaced untouched MPG files look fine but definitely shaky when played in VLC, but every time I deinterlace using QTGMC I'm left with this weird phenomenon where specific parts of the frame seem to jitter back and forth.

I'm using StaxRip for reference, but wouldn't mind pointers for AviSynth as I've been meaning to properly learn it for a while.

https://imgur.com/a/YCuOyW9

It's most noticeable on the character on the right (James) face in the flashback scene, is there an actual name for what's going on here, and what would be the best method to get rid of it?

[geometer]

My VHS cassette player did that all the time.
I think what added up was mechanical instability in the capstan and rotating head system,
interfering with the interlacing mechanism,
and probably intermodulation that warped brightness vibrations into timing jitter.
These issues used to be very signal dependent. It was in the hands of the content producers to trigger not too much of that.

[Sharc]

..... but every time I deinterlace using QTGMC I'm left with this weird phenomenon where specific parts of the frame seem to jitter back and forth.
Did you set the field order (TFF or BFF) correctly before throwing it at QTGMC?

[DominoDeer]

Did you set the field order (TFF or BFF) correctly before throwing it at QTGMC?

I tried TFF and it gives me the same effect/result, doesn't seem to fix it at all. BFF gives the footage a stutter like some of the frames are being played out of order.

[pbristow]

I see the poor engine's face quivering up and down, and at first I thought that was a deliberate animation to show how miserable he is! However, the effect is also present below his face - shows up clearest on another round feature, his buffer, and also on part of the ground in lower left of picture.

I think the main problem here is that whatever's doing the image stabilisation (is it just the de-interlacer?) is working on different parts of the image separately. Vertical jitter in video-tape sources is better handled either as whole-frame stabilisation, or - if parts of each frame are frequently getting corrupted to the point where the upper and lower sections of the picture aren't being accurately placed relative to to each other - on a line-by-line or stripe-by-stripe basis.

Sadly it's so long since I've been a truly active Avisynth user that I can't be more specific about what tools are best/available for doing that. (In my day, it involved rolling up your sleeves and getting your hands dirty with MVTools. =:o} )