Hi,
I just bought a brand new RB1200 router , we have just 120 pppoe active clients but rb1200 cpu usage cross 70 to 90% 
is this core router ??? very low cpu i think ..
???
what else is running on it?
how much traffic? check if your router is not being attacked, or if some of the clients is not running some trojans or botnets
i email you at support@mikrotik.com from info@webxs.net.pk
kindly see the support file
Also make sure you are having global change-mss mangle rules instead of many individual ones.