Lag hvert 3-5 sekund

I have a custom made pc, it contaions a i7 cpu 930 2.80ghz, and a gtx 670. I've been experiencing a consisntent amount of lag. It seems as if the whole pc lags for a less that a second, and the lag is including everything from image, input, output and most annoyingly sound. Every time i listen to spotify it freezes every third og fifth ish second and it is rly fking annoying. So my issue is that i dont know the cause of this issue, i've tried follow a few videos where you go in on software and fix some things, didn't do anything, been resetting my pc as off writing this and i now am pretty sure that it is my hardware and not software.

My brother says it might be the motherboard but i don't know what type i got, but since my CPU is only about 143DKK i think that might be the issue.

But before i buy anything i'd like to get some advice before, since im not Bill Gates or some rich dude.

Thank you for reading this through and i appreciate any sort of answer. (even stuff like "lol gey" cuz that shit funny.):)  - på engelsk fordi jeg skrev først på en engelsk hjemmeside men man ku' ikke logge ind der fra Danmark eller noget