What CPU are you using?
Ensure you have plugged in the CPU fan properly, in the CPU_FAN section of your motherboard, all the way in.
Try turning up the RPM of your CPU fan through your motherboard settings.
Try and clean your computer and heatsink, with compressed air.
Re-seat your heat sink and thermal paste.
Here is a guide on reseating your heat sink:
https://www.youtube.com/watch?v=Q2p6Hk4IfqI&feature=c4-overview-vl&list=PLQMVnqe4XbictUtFZK1-gBYvyUzTWJnOk