Ab, thank you for your post.

My coworker was on vacation that week and when I described the problem to him, he remembered that he worked on the issue in January 2012. Apparently, the upgrade to SP3 caused more memory to be used which limited the number of threads available to IBM HTTP Server (IHS). Other occurrences of the problem caused IHS to issue messages that it couldn't create a new thread and that it was performing a graceful restart. Several of the graceful restarts kept the web server out of the rotation for up to 40 minutes (for the work to clear out).

We received some apache tuning recommendations from IBM (to increase the number of threads) and we increased the virtual memory from 128 MB to 256 MB. So far, the problem hasn't reoccurred.