I'm having a hard time making heads or tails of the Site Speed graphs in Google Analytics. Our site speed is fluctuating wildly from month to month, despite a large sample (the report is "based on 100,000's of visits) and a consistent web set up (static files served from an EC2 instance running nginx behind a load balancer).

Here's our site speed, with each datapoint representing a week worth of data. Over this time period we modified our source and HTTP headers to increase our cache hits on static resources by 5x.

Why would it fluctuate so much? Is there any way to get more reliable information from those graphs?