Xbox Lagging on one of my TV's, Help Please!?

When I play on my one TV, a 26" Sanyo (HDTV), my games lag just enough to get very frustrating. I read something about converting incoming signals or something causes the lag. My Xbox is connected with an HDMI cable. I have tried changing from 420i up to 1080i, but none of the resolutions make a difference. I also did display discovery turned on, and optimal resolution on, but no luck. I tried the Red, Green and Blue HD cable also, but that didn't change the lag. Is their anything I can do to fix it, or is my TV just crappy. I know it's cheap, and not very good, you don't have to tell me. Oh, and also, their is no lag at all on my Panasonic 32" HDTV. Thanks for the help!