Try a profiler run to capture poorly performing statements. Also use PerfMon and Task Manager to capture performance metrics. Your indications suggest CPU issues. Find bad query(s) and tune appropriately. One possible thing is out of date stats leading to bad plan. Another is someone dropped an index (or one is needed due to data skew, etc).
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru on googles mail service