That sounds like a HDD bottleneck, What Hard Drive are you using ? I get a slight lock up using an SSD but I assume it because it saves before quitting....
You say complete campain levels, Im still on the 1st main campain, Have you gone past this.... Ive only been playing for 12 hours or so...
Your CPU is fine the higher the better 16% isnt high, RAM usage isnt high either..... By rights the better the game is improved the more ram it should utilize.... On an 8 core cpu you should see each core with the same approximate usage.... sometimes less depending on whats being calculated....