Streaming and Gaming problems

Ive been trying to stream on Twitch with Open Broadcaster. I put my streaming settings to the best performance but still keeping quality 720p and 30fps. Ive turned Black ops 2 to the lowest settings. The only stuff I have open is Black Ops 2, Open Broadcaster, and iTunes. Now when im streaming its starts out good at 60fps cause I have it locked. But after some time it starts struggling. Like I drop to 30 frames sometimes and itunes starts skipping and stuff. I overclocked to 3.6ghz. When I check task manager my CPU is only being used about 23% and Memory is 45% and Disk like 10%. Whats making my computer struggle? Cause its like it should run a lot better