PC sticks for 15 seconds then frees up for 3

My mouse seemingly randomly sticks like the PC has crashed for 15 seconds and then it can move for roughly 3 seconds and then sticks again. 1 week ago it was doing this and so I hard reset and then the motherboard blew. I have replaced it and put more memory in and today it is doing it again (but not blown the motherboard when I reset this time), the power and processor lights stay illuminated but I can't hear the processor clicking away like it is doing something stenuous. What is doing it? I have Bitdefender and it hasn't picked up anything nasty.