Video lag on 1080p tv?

So I have two HD tv's that I play xbox 360 on, one upstairs, and one downstairs. The tv upstairs is capable of going up to 720p ONLY, and when I play on it, I have no video lag what so ever, the controls are perfect and a ok. But downstairs, the tv can go all the to 1080p, but even when I switch it down to 720p, I have pretty bad video lag. Is there some setting i have to change on the xbox oe the tv to get rid of the lag? I use a HDMI cable by the way.