All, I have searched MSOfficeForums as well as other sites for a solution to this issue. When I noticed the sporadic problem occur, the only solution was to save, exit, re-open. Then, Find would work for a short while, then stop. Yes, I tried all of the suggestions, such as look in values, match case, match entire cell contents, but no luck.

Finally, I dropped the entire spreadsheet into Notepad and pasted back into Excel. Now Find works properly. The data was generated from a mainframe ASCII text file (yes ASCII, through SAS), then imported into Excel. This may be part of the problem, I don't know. Anyway, this seems to be the ONLY solution.