When you get the random lag attacks, ALT+TAB out of the game and open the Task Manager (Right-Click on the Taskbar and select it) Now hit the "Show processes from all users" button and see if there's a program other than Halo 2 that's using a lot of CPU power. Then attempt to close it
NOTE, if it's Trustedinstaller.exe then you need to shut down the updating service through the Service Control Panel (Start->Run->Services.msc)

Also Read the Sticky PROPERLY and post a PROPER DXDIAG.