i think i remember having to switch the cable when i played H2. i defo remember having to do something. i know there is a reason why i cant be bothered playing H2 as regularly as i used to anyway.
here is what i think it was, i will be able to go check in the morning as the xbox is in another room now.
plug 360 in with std av cables, remember to flick the switch on the output connector.
go into the xbox settings page, and set your output to 60hz.
now switch back to hdtv on the connector and i think it should work.
as i say, i will check in the morning and post if it is any different.