Website trying to pull up a page from a different site

We've had an issue come up a couple times in the past month or so, and I don't know what is causing it. After the first time, we wrote it off as a fluke, but it happened again today, so I'm looking for an answer.

One site suddenly starts accepting requests for another site. For example, looking through our tracking, a user hit domain1.com/pagename at 2:09:07pm but then hit domain2.com/pagename2 at 2:09:12pm and all hits after that were sent to domain2.com even though the pages reside on the domain1.com.

I have a log of what the PHP _SERVER variable held when I got the 404 errors; the SERVER_NAME shows domain1.com while the SCRIPT_FILENAME shows it is hitting domain2.

I have looked in the IIS logs, but it just starts giving 404 errors for the pages that don't exist on domain1.com.

When it happens, all the sites on the server go down, and we have had to restart the server to get them back up.

Not sure what to think on this one. Could it be a RAM issue, App Pool issue, FastCGI issue? Any help would be appreciated.

[edit] So, it happened again. This time, it was the middle of the night, but we were not required to restart the server; it somehow fixed itself. However, I got the following error from FastCGI: The FastCGI Handler was unable to process the request. The FastCGI process has failed frequently recently. Try the request again in a while Error Number: -2147467259 (0x80004005). Error Description: Unspecified error

Also, it is always the same two sites that seem to run into this issue. We have made sure they are using different application pools. I don't know what would be causing this.