typically i get slow downs when the cpu is too hot or a process is running overtime or the same process is taking up a huge amount of memory. Take for example f1 prix world manager game, their is an editor also a useful cheat however when your finished editing in the problem and switch it off, the window closes but the progam is still running so you keep editing with this program and shutting down until you come to play something else where the whole pc is running around bending backwards because its running at an odd speed. make sure that before you run anything memory intensive that you check all the processes. i have found an isolated problems with games before by leaving windows process manager open (cntl + alt + del > processes tab) while playing a game, another way is by going through tune up utils and it will take down your actual free memory at any given time once you active the program.
failing that, check every and all parts of your machine for a stray bit of dust or moisture build up in any of your connectors to your parts, to also be sure to clean out your psu, you really dont know how much dust collects besides i never discount the possibility of dust on the psu, and if you hadn't realized, if dust collects on your cpu cooling unit the cpu cant to its workload because of a cpu meltdown likewise the psu, beside that unit processes your power and will slow down which can not deliver your pc the power it needs, hence lower voltages meaning less power and so everything slows down, make sure your pressure clean out the certain parts for any dust.
