I have some similar problems
lemme give you my situation... I have a few nvidia cards and a us robotics branded hauppage bt848 card... and for me to be able to capture using my bt848 i have to:
1st load the module at boot before nvidia's driver.
2nd i have to unload and then reload bttv after Xfree86 is already running!!!
3rd capture all day long
I hope this helps you... please post us a follow up so we know how it turns out