1. Check that the case fans are working and they're creating a good airflow. The basic layout is a fan on the front bottom of the case drawing in cool air, and another fan on the upper back moving out warm air.
2. check that the PSU is not too hot
3. check that the CPU and GPU fans and heatsinks are free of dust
4. make sure your PSU is powerful enough to feed all the components (including hard disk, optical units and fans)