Why do some of my games only SOMETIMES run slow?

I recently installed Need for Speed Rivals on my PC and though my PC specs for gaming is pretty decent, the game sometimes runs slower than usual… Lagging occurs pretty rarely, but in general, the game just runs slower than usual (it's like there's a half second delay between the time I push a button on my controller and then see the appropriate response).

There are also times however when I boot up my PC when the game runs perfectly without any of the aforementioned problems.

My specs are as follows:

Lenovo Ideapad Y580
Windows 7 64 bit
Intel i7 @ 2.30 GHz
8 GB RAM
NVidia GeForce GTX 660M

I performed an antivirus scan and ran the defrag several times since and ended several high memory load processes before using the game to see if the performance would improve, but it didn't affect anything in the slightest.

As I said before though, it DOES run perfectly sometimes, but I have no control over knowing when those times are or what's causing the game to slow down during the other times.

Any insight would be appreciated!

Hit Ctrl-Alt-Del as soon as your game starts to slow down, click on Start Task Manager, click on the Processes tab and sort by CPU, check which active process is taking an unusual number of CPU cycles.

Run msconfig and disable any useless startup apps. Which apps to select is up to you. Make sure to google the executable file names and research these files before disabling.

http://netsquirrel.com/msconfig/msconfig_win7.html