Have you checked on the site for CoD2 for patches? It's possible that there have been some patches released for CoD2 that will fix the problems you're having. You also might want to tweak the video settings of the game. It's possible that you're using software rendering for the video which taxes the CPU.
• Apple iMac G5 17" (2 GHz G5) - Mac OS X 10.5.8/Ubuntu 10.04
• Asus Eee PC 901 (1.6 GHz Atom N270) - Fedora 13
• Apple Macintosh Quadra 650 (33 MHz MC68040) - Mac OS 8.1
• "JHVH-1" (2 GHz AMD Athlon XP 2400+) - Slackware 13.1