I found the issue, was a bug in my miner. Moved to an older version, problem went away. Doh.
All of the latest miners use 100% cpu, seems like some sort of bug in the engine or mining code. If I swap to slightly older versions the usage drops to < 2%. Any idea what might be going on there? I'm running the latest drivers as of today.
For aggression/intensity, I"m using 14 under BFGMiner, and 12 aggression on phoenix.
I think the newer drivers introduced the 100% cpu bug, but only at high aggression/intensity. For me (driver 12.
, I:9 is good, but I:10 or higher isn't.