the first time i set up a new frame with disc brakes i spent most of an afternoon screwing around with the brakes. i was having problems exactly like what you're describing. the calipers were rubbing pretty badly most of the time.

since i had a good shop within walking distance and some errands to run i brought it down there. The mechanic got it all fixed for me - he said the disc tab wasn't aligned, it was off a little and that was causing the problem. he prepped it, did a small amount of filing and the brake's been good since.

so much for QC