I've been using the Certificate Patrol add-on for a couple of year and I've noticed that there are periods of time during which Google's certs change very frequently (at least as seen by my browser). The last week or two has been one such period. It seems to be switching between certs from Equifax and GeoTrust multiple times per day.

http://patrol.psyced.org/

I've been thinking about installing the EFF's SSL Observatory - a sort of distributed cert pinning and comparison plugin.

https://www.eff.org/observatory