Wrong Last-modified header for blog RSS feed

I’m having a problem with the feed from my blog returning a stale Last-modified header, and I’m out of ideas for how to get it to update. I have my AWeber communication account set to read this feed for updates, but after initially setting it up, and sending the first broadcast, the server’s Last-modified header will not advance from that date (May 26), and I made several posts in June that should have triggered it to update.

I tried updating from 2.9.2 to 3.0, but the problem persists. I even have tried disabling the WP-Events-calendar plugin, which warned me when I activated it in the 3.0 install that it may cause issues with syndication.

I can provide more details about the install if necessary, but I am basically running 20 plugins, and a customized Kubrick theme, hosted on GoDaddy. Please help, or point me in the right direction if the solution is already known. Thanks!