[SmG Nukem]

Sounds like something running in the background on your computer may be the problem.
Start task manager and leave it running. Have a look at the usage while you are playing to see which processes are using the most memory. Check the CPU column for the processes that are using cpu resources.
If you are running optic then be aware that it can leave processes running even though you think that you have closed them.
Do you shut down your computer or leave it running all the time? If you leave it running then old processes accumulate over time.