Applying the brakes at higher speeds

[AuctorEcclesiae]

Hello friends:

I've been meaning to post this question for awhile now. I had new front pads and rotors installed in mid-April, and I had the rear rotors and pads inspected. They were fine, so no replacement was done on them.

The brakes are great -- the pedal is nice and firm and I have no problems with them. If I apply them gradually, they don't really have any problems.

My problem arises when I'm going 55-70 and I press firmly on the brakes. The whole car kind of vibrates or shimmys -- that's the best word I can use to describe it. It only does this when I'm traveling at higher speeds and have to press firmly on the pedal. If I press the pedal gradually, there is no vibration.

Would anyone have an idea as to what might be causing this?

Thanks for the help!!

-Michael.