I've seen similar wobbles in GPS-disciplined standards. The fundamental
gradually drifts off then the GPS software algorithm slowly pulls them back
in again. How often and how far depends on the programming.
Lots of them in use now. There are four on the cellphone transmitter site
next to me - amusing watching their harmonics on 144.000 000 MHz wandering
back and forth.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.525 / Virus Database: 322 - Release Date: 09/10/03