Warped discs. (or a high spot on the discs, same difference)
Light pressure allows the warp to show itself and heavier pressure keeps the pads in the same place and the judder transfers through the disc pins.
Bear in mind it might only one of the discs that's warped. Take the wheel out, remove one of the disks and put the wheel back in and check for the judder. If its gone you got it. If not, Drop the wheel and prise the bads back and remove the other disc and replace the one you took off. Try again. Gone?
Temember to pump the pads with the lever before you move the bike each time to push the pads out .
Or its a poodle under the seat, scratching as you brake
C