Hello Gordon,
I think valuable clues lye in the fact that this can be felt through the brake pedal, and that it seems to have disappeared for 2 months ( the longest symptom free period) when the brake discs and pads were renewed.
What make discs and pads were used?
I ask the question since your description smacks of symptoms of warped brake discs.
If this were mine, I would be wanting to check the run-out on the discs as they are now, in their installed state, and if found to be warped, I would be extremely fussy to ensure the mating surface of the hub - where the disc fits - is scrupulously clean and sound. If genuine VAG discs haven’t/aren’t going to be used, then I would opt for Brembo, and not be very keen to use anything else. When fitting new pads, I would be again be very fussy to check that the caliper sliding pins were clean, suitably lubricated, and we’re moving freely.
All of this would be my starting point, unless anyone else sees anything untoward on inspection.
Kind regards,
Gareth.