My Fifa 06 isn't running smoothly though i have 128 mb agp.?

Hi. I'm playing Fifa 06 in my desktop Pc which has a 512mb of RAM, Intel Dual Core processor 2.6Ghz and internal 128mb of video RAM (agp). I'm using Windows xp professional sp 2. When i play the game it runs smoothly for 5-6 seconds and then slowly 3-4 seconds. This thing keeps repeating. Its really annoying when it becomes slow at the time of giving a goal. I think this can be cured. Can anyone help me?