We've had several threads about this in the past and there's never one solution that works for everyone. One past thread is here:viewtopic.php?f=33&t=13776
but any mention of patches should be revised to the 1.10 patch, not 1.09 or older ones.
Are you running Halo in a window, or fullscreen mode? Which ever one, try the other to see if it makes a difference. What resolution do you have Halo set for and what is the actual resolution you are running on your monitor (if you are doing fullscreen)?
One patch you should install anyway is Hac2, and it may help since it does revise chat.viewtopic.php?f=33&t=25830