Any Suggestions About Part Time Judder?

[Skraggy_uk]

When I test drove the IS there was a judder through the steering wheel arround 50-60. It seemed to go off after while, but dealer said he would get it balanced.

Before I collected he it had been balanced by local Hyundai dealer across road and he had tested it upto 65 and there was no judder/shimmy/wobble.

That was Friday. Yesterday was the first time I've been over 30 since I got it, and going over M6 Thelwall, about 20 minutes after I set off, at 50 5th seemed sluggish, 6th wouldn't pull at all, could barely hold revs. In 4th and 5th at arround 3k ish revs the vibration started again. Enough to rattle the keyring against the dash, and if I take my hands off, blur the wheel. You can actually feel the whole car shimmy, and feel the vibration through the seat first as the wheel starts to shake. It also pulls very slightly to the left.

10 or 15 minutes later, everything fine at all revs, and will pull cleanly from 30 in 5th and 40 in 6th.

I need to take it back to get some stone chips sorted, but what are anyone's thoughts to the cause of the wobble. I want this sorted while there is a warranty.

Is it likely to be prop or gearbox mount, wheel balance or sticky caliper, or a disc/hub face (it has had at least one set of discs/pads). Braking feels smooth and doesn't pull particularly either way. Another thing is, when stationary at traffic lights, the car is rock solid doesn't roll, no brakes, out of gear. Whenever do you find junction anywhere in the UK that is that flat? I've had it at every set of lights I've stopped at.

It really confuses me how it can be so bad, then 10 minutes later be so smooth.