Look at your power savings plan.
Set it to high perf or what ever it's called.
Then go through each option in the plan and adjust.
Firstly, i have recently build this pc and it has only had this problem for the last 3 months or so, the pc itself as a whole is less than a year old, other than the graphics card and the power system, they are about 2-3 years old.
Processor: intel(R) Core(TM) i5-6500 CPU @ 3.20GHz 3.20GHz
Ram: 8gb kingston HyperX ddr4 fury
Graphics: Nvidia GTX 760
OS: windows 64bit
the strange part is i have left the computer running multiple times with task manager up and waited 10+ minutes for this fps drop to happen to just have a look at whats happening but it refuses to drop fps while task manager is active, this sounds like a simple fix right? well while watching movies i cant exactly have task manager on the screen in the way because the frames will drop if its just running in the background, i don't know what to do to fix it, there is basically no dust in my computer everything should be running fine but the fps drops are killing me, if anyone could shed some light i would be grateful for any advice ( iv updated graphics card drivers 3 times or more since this has started happening, one thing i haven't tried is going back to old drivers but i dont know how that would effect gameplay or if its even the problem since its happening regardless if im gaming or not)
long story short i cant watch a tv show or movie on my pc for more than 2 minutes without having to get up and move the mouse.