Take a look at Brent Ozars article on "playing doctor".

It lists a bunch of free scripts out there to help identify problems such as high CPU.

http://www.brentozar.com/sql-server-training-videos/playing-doctor-with-dmvs/

Take a look at the script from Glenn Barry. It has a ton of diagnosis queries. There are two queries you want to search for. I enclosed just the description since I Glenn did all the work.

A - CPU

-- This helps you find the most expensive cached stored procedures from a CPU perspective

-- You should look at this if you see signs of CPU pressure

B - MEMORY

-- This helps you find the most expensive cached stored procedures from a memory perspective

-- You should look at this if you see signs of memory pressure

Good luck on finding your issue.