Those of us who have felt the feeling of warped rotors know it instantly. I can feel it now in my foot from the car I test drove in 2001. Fortunately, my car has never had warped rotors, because when I started reading this board, I faithfully torque my wheels. I've stopped pulling out my torque wrench in front of various service people though. Sometimes they say 'oh yes, this gun will do that' and I get home and it's like 300 ft/lbs.
The other shudder I had experienced was alignment and tire balancing. But you said that you had been to an alignment shop. I'd still bet on the rotors though, and the speed of it would change as Klaus says. Maybe if you imagine a warped record on a turntable, and your foot is the needle. (assuming you're old enough)
Good Luck
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'97 855na, Mobil 1, K&N, various IPD's and eBay's