Mm... Do you have any autostart apps that impact the performance and boot up speed of the PC a lot? Autostart apps are really annoying when it means you want more performance. Especially the ones that impact the performance a lot! Try disabling unwanted startup apps. Also. Did you install any new application? Check for viruses/malware in your PC. Try checking your hard disk usage, it might sometimes be the cause. Maybe the hard disk usage is high because of some process running which will impact performance a lot!
Also, do you experience this problem with other resource demanding games?
Hope you get it fixed!