HI Barry...welcome to the Forum!
That sounds very odd as if it was warped discs it would occur all the time, albeit with different feedback at varying speeds.
Same with wheel balance, which will always occur at around the same speed and you can drive past it by going faster.
But your problem is strange if its intermittent...would check to see if anything is loose as opposed to worn (if it was worn the fault would be there all the time)
Keep us informed on how you get on with it
Cheers Trevor - Admin