Increasing FPS. Suggestions

[OneEightyy]

I have an HP computer with an AMD Phenom 8550 Triple core 2.20 GHz 4.00GB of RAM. Nvidia Geforce 9100. In shooting games I get very speratic lagg spikes. Around every 5 minutes I'll get a very bad lagg spike. It was running perfectly fine earlier. Just started happening recently. Suggestions? I plan to eventually buy another video card eventually. Whatever you think the problem may be, try and keep the cost under $600.