If it is only when the brake are warmed up then I too would suspect a bad rotor. Any good suspension shop should be able to rule out most other causes such as bad steering rack, bad tie rod ends, worn suspension bushings, etc.
Although unlikely, it could be dripping brake fluid on the drums, but that is a stretch of the imagination.
New rotors do warp, and this is more likely with cheap after-market rotors and those which have been improperly installed or over- torqued wheels.
You might want to try cross-drilled rotors which dissipate heat more efficiently and reduce the chances of warping.
__ __ Randy & \ \/ /alerie's \__/olvos '90 245 Estate - '93 965 Estate "Shelby" & "Kate"