2005 Pontiac Vibe Rotor warping problem?

I am having trouble with the brake rotors on my little Vibe. Every 5 or so months, the front rotors seem to warp and cause severe steering wheel shake upon light braking. I say "seem" to warp because in order to cure the problem, they only need turned slightly and then it's good for another 5 or 6 months. Sometimes they don't appear to have anything wrong with them, but they still behave as if they are badly warped.This happens usually in warmer weather and on longer trips. I have checked the calipers for a problems and bled the system just in case one of the calipers wasn't working properly. That didn't help. Can there be anything else I can check since I am tired of taking the rotors off and having them turned. I have also bought a new set of rotors about a year ago and they also developed the same problem. In fact, today will be the second time I've had these new ones checked and turned. Is this another case of bad GM rotors? I thought Vibes were Toyotas? Thanks for any info as I'm pretty much out of options other than ordering a set of drilled rotors so that they dissapate heat better. Val.