Why do i lag whenever i enter combat in wow?

Whenever I enter combat in wow such in a duel or in pve whenevr i go kill a boss or a mob i get huge lag i know it isnt my internt connection or the server but i checked on task manager whenever tht happends my cp usage on wow arises from 10 to 50 then my wow window crashes.This has happend many times idk how to fix it can some1 tell how to fix this?