Intermittent DNS drops with Mavericks 10.9.1
This is not a solution, per se; but possible further refining of the problem.

I have an iMac 7,1 4GB with 3 different external system drives - 2 10.9.1 (1 primary, 1 backup) and 1 10.6.8.
The 10.9 I used as primary system started dropping the wifi connection randomly; but repeatedly. 
Running a ping in terminal made no difference; neither did downloads in progress. 
I even moved the router to 2 feet from the computer and there are times the computer does not even SEE it.

It DOES NOT happen with either the backup 10.9.1 or 10.6.8.

Tried running Wireless Diagnostics - not sure what to look for; but logs just say dropped.

I tried reinstalling 10.9 on primary (without initializing drive) - no joy, so just doing a basic reinstall hasn't help me.

I have either downloaded/installed something or parts of the communication framework have been corrupted somehow. Whatever it is, it is something that is not repaired with a basic reinstall or something that the diagnostic tool makes obvious to me.