It is uncommon, but it is possible for rotors to cause an intermittant vibration, especially if it happens when the brakes are hot.
In that case, the rotors don't warp, but rather they gain "thickness variation" when hot and some of the metal expands more than other parts of it. If there is visable "micro cracking" on the rotor face then this has happened enough to cause the rotor to start to come apart. If the rotor has "micro cracking" or is gaining thickness variation when hot, machining the rotors will not help. The rotors will need to be replaced.
I also agree with the above postings that it is critical that the brake pads have shims (or brake quiet) and that the caliper slide pins be properly lubricated. A stuck caliper slide can sometimes also cause brake shudder that feels just like warped rotors.
