CPU Lag

I have a problem with my computer everytime im using
anything playing a movie and games every 30seconds to one
minutes a lag happens for 3-5 seconds and its really
annoying. So i opened the windows Task manager and i went
to the cpu ussage and i see that the cpu ussage flucuates
every 5 blocks to 100% then regualates at 15% for another
5 blocks and does the same. It has not been like this
since i got the computer. Please help thank you