Sounds like you have another application intrupting your game time.
This can be real tricky to fig out what app is doing it on your computer. try shutting down any IM programs like msn aim yahoo ext ext. some antivirus programs will also do this.
so basicly try shutting down anything your not using at the moment play your game and see if it helps.
press ctrl+alt+delete, click on the "processes" tab. list everything you see there. then go to start...run... type "msconfig" without the quotations. click on the "services" tab. list everything you see there.