A bizarre text problem

Although I work in an office with linked Macs, my particular terminal isn't new, and not particularly up-to-date (Mac X, Version 10.4.11). I mostly use Word. I use Firefox for Internet, but have frequent freezes necessitating a "Force Quit."

While editing a document I've been working on since around 1998 (so it's gone through several versions of Mac and Word), I noticed that several patches of text are corrupted. When I clicked the "¶" toolbar button (show nonprinting characters), the patches of text were displayed as being overlaid by ÿÿÿÿÿÿs (y with a diaeresis). Clicking the ¶ button to hide the nonprinting characters made the ÿs disappear. The affected text looks absolutely normal, except that it's peppered with tiny squiggly red lines indicating misspellings, but these are only one character long, whereas with genuine misspellings, they underlie the entire word. Doing a spellcheck yields no helpful results, as the words are not really misspelled, although various letters are displayed in the correction-suggestion pane.

But copying and pasting pieces of text in the "Find/Replace"search bar shows that the letters of each word have spaces between them (which I certainly didn't put there). These spaces don't show up on a regular search, and they can't be corrected automatically in any way. The only way to fix the affected text is to retype it from scratch.

Can anyone explain what may be causing this, and advise me on how to fix the text without retyping every affected patch?