Mavericks clock running fast

Since my Mavericks upgrade, my 27" iMac clock runs fast. It was about 3 min fast today after a few weeks since I reset it last.

If I open my Date & Time Preferences it corrects the time, but then continues to run fast.

I have it set to: Set date and time automatically: Apple Americas/ U.S. (time.apple.com)

I do not have a check to set time zone automatically, but chose the city.

Any suggestions?