Thanks for the info, Tim. I had already increased the memory size prior to the issue. It hadn't changed the performance of the offending plug-in (still lookin' for the one, assuming it is a plug-in error).
Andrea_r: Thanks for the link. I had read that article specific to wp & wpmu a number of days ago. No offense, but I did not see where it addressed any specific differences between writing a plug-in for wp vs wpmu. I keep reading articles and forum posts, however, that state there is little or no difference writing a plug-in for wp vs wpmu.
Nonetheless, the difference I see is that each of the active plug-ins are running on both a wp and a wpmu install (plug-ins are identical, i.e., versions, etc.), where there is no issue on the wp install, only the wpmu install. So there has to be a difference in the way wpmu incorporates these plug-ins vs the way wp incorporates them.
For example: It could be something as simple as the time required to query each of the wpmu sub-domain database tables vs. the single wp database table, or an internal function call specific to wpmu and not wp. I don't know. I do know that I keep reading about other people wanting to know about a wp/wpmu plug-in compatibility listing. I do not understand why such a listing would be requested if no differences existed.
I have been a programmer for over thirty years, so I eventually will find out what is causing this problem. Whether it is due to a server configuration setting, improper function calls, pilot error, whatever, makes no difference to me. I'm not looking to assess blame, rather to find a solution and share that solution with others.
I welcome any and all suggestions and I appreciate the help and the time y'all have set aside to assist others on this forum.