What happens if you move both windows to one monitor? I was experiencing choppiness with my 5 box setup [1x Character on main monitor, 4x Characters on second monitor]. I moved all my windows to my main monitor using Keyclone or InnerSpace's window layout and my frames were magically better. I read [no technical source to give you right now] that Win XP 32Bit has limitations on dual monitor setups and that 64bit OS's do not. So because I did not like everything jammed on one monitor I decided to upgrade my OS to Vista 64 bit and everything was perfectly fine. I have since then gotten the new Windows 7 RC1 64bit and it's running great.

Here is a post from the DB forum that maybe perhaps will help you out [last post].
Win XP, multiple vid cards, and 2 different monitors