Ok, recent visitors turned off for the weekend and no complaints from the users. Which I assume is good, since I'm sure they'd complain if they still got the error.
Going to start checking hacks this week as I am now seeing the error more frequently on my test board, which is 99% identical to the live board.
The only question is, what is now.cgi used for? Just to show recent visitors? Must be a mistake I made in WOL hack... only thing I can think of... Will have to dig out my list of hacks and see which ones affect recent visitors. hehe
I will report back if I have any luck.

Thanks for the help, suggestions, and ideas everyone.
~Z