I had something very very similar to this and at similar speeds. At first i thought it was warped discs, changed them along with new pads and still the same. It was tracked down to the inner tie rod ball joints. Both sides had play in them. Once changed ( I used VW sharran ones made by MOOG ) it went.
PS: I mention the make of inner tied rod I fitted this time round because the time before that, I fitted TAZU ones and they didn't last long at all. If you do change yours for VW/an alernative one(s) check the thread length that screws into the rack.
see post 2 here for alternatives.
http://www.the75andztclub.co.uk/foru....php?p=2265397