PT 11.3 System usage increase dramatically when idle

[Phil Buckle]

Just installed 11.3 on a new MacBookPro 2.8 with 16 gig ram using an Apollo 16 interface. mavericks.
Loaded up the demo song that came with the download.
Whilst playing the CPU usage showed around 20% right to end of song.

When I hit stop and wait about 3 minutes with the computer doing nothing the CPU suddenly rises to 50% and stays there.

My system usage meter shows 8 separate bars and it's the 4th one that goes to 50% and of course so does the overall meter.
If I quit PT and reopen the meter goes back to 20% until I stop and wait and it rises to 50% again.
Reminds me of the very "old" days when PT had the odd memory leakage.

Don't even know if there is such a thing any more.

Any ideas?

[stargazer]

I don't see anything like that on my Nehalem Mac Pro 2x2.66 GHz Quad-Core.
What I still see in my Activity Monitor is "Pro Tools Quick Time Server (Not Responding)", but it doesn't seem to influence anything.