Are resource_semaphore_query_compile waits causing 100% CPU on server?
Have you checked active tasks too, in addition to waiting ones? Check sys.dm_exec_requests for offenders with high cpu_time values. Look in sys.dm_exec_query_stats for queries with high total_worker_time. 

An explanation for what is happening could be that a plan changed to result in a very CPU intensive execution plan with a large memory requirement (eg. hash joins, sorts). Parallel table scans and joins on non-aligned partitions come to mind... Few queries execute but they consume high CPU (the query exec and query stats views will reveal this). The queries require a large memory grant and cause other instances of the same query to block on the resource grant. A look in sys.dm_exec_query_resource_semaphores will reveal if this is true. Solution would be to address the plan stability, perhaps deploy plan guides.