Sounds more like hot-spotting to me and friction material transfer from the pads. Warped rotors are highly rare in passenger cars, as it take extreme heat to cause this. This would also explain why its worse at times.
What happens is that your rotors get extremely hot in one area, then the friction material from the pads sticks to the hot-spot, leaving a small amount behind, causing high rotor run-out.
It varies, as sometimes the material will shear off of the rotor and the shuddering feels less. It turns into an endless cycle after a while.
There is no rhyme or reason to why it happens, but substandard brake pads are typically the culprit. I always recommend high-grade semi-metallics or ceramic pads to help combat this.
Wednesday, March 2nd, 2011 AT 2:23 AM