Could be an addon, could be a background process using CPU, could be overheating. Very hard to tell without any diagnostic information to base troubleshooting on.
What to do if you need more help
Technical issues can be very frustrating to try to diagnose on your own. Fortunately we have a helpful community that can help locate the problem with you. If you prefer to troubleshoot with the community and see if anybody else is having similar issues, start a thread with this information:
For all technical issues:
Server Being played on
What Video Card you are currently using.
Location in the world if you are having connection issues.
ISP if you are having…