Huge traffic drop after vB 3.7 + vBSEO 3.2 upgrade

On July 24th I upgraded my cooking site (link) from vBulletin 3.6.8 to 3.7.2. The same day I upgraded vBSEO to the latest 3.2.0 release. I was reveiwing my stats noticed a 75% decrease in unique visitors and page views are down about 60%. Following are links to 2 publicly viewable stats on this site:
Quantcast
eXTReMe Tracking

Here you can see the public announcement about the upgrade on the same day the traffic started to nose dive:
Upgrade Today (July 24, 2008) - Discuss Cooking Community

I am afraid to make any changes on my other sites after seeing these results. Do you all have any suggestions on what I should do to find out what might have caused this drop in traffic?

Well I just logged into Google's Webmaster Tools and it shows a ton of errors. Attached is a screen shot of the errors plus a zip of the CSV download of the errors in the Sitemap.

When I click on the links that Google says are not working they work fine for me. I can't figure out why all of a sudden these valid links would return 404 errors to Google? What can I do to clean out Webmaster Tools and get the URLs listed again?

did you have server downtime recently? I've noticed bba being very slow occasionally, then super fast 5 min later. Google will 'timeout' if your site takes too long to reply.

Google will fix it self.

It should have a 'last accesed' date associated with each 404 record.

Are there groupings or dates that seem to be the culprit, or they spread out?

These show up as 404 errors. I can see some other errors on a different site that are under "unreachable" category so I think these are getting a 404 for some other reason.

Regarding grouping of dates, they are spread pretty evenly between July 24th and August 2nd. So there does not look like a single day where this could have been an issue.

I am going to bed now but plan to keep trying to figure this out in the AM.