I have the same issue, front disks and pads were replaced a couple of weeks ago so hope that is not the issue. When I get the chance, i'll take it back to the garage to see what they say, hopefully something simple (and covered under their warranty!), she's only 2 weeks old!
Unfortunately i had to go into work sat morn and by the time i got out the garage was shut. SAying that while i was reversing down the drive and applied the brakes there was a jolt, and the symptoms were reduced when braking after. I think maybe something lose ?
It will be the brake discs. Very common on all cars to get wobble under braking, especially as the discs thin with age and use. They wobble when they heat up.....like when braking from speed, or braking hard.