Unchecking strikethrough doesn't always "take"

[Bob Gardner]

In editing, I have occasion to go to Format, Font, and check the
Strikethrough checkbox. After confirming that the text was indeed
"strikethroughed," I uncheck that box and begin entering text again. Like
all good touch typists, I don't look at the screen all the time, and
occasionally when I glance up at the new text I find that dratted line
through it. Going back to Format, Font, I see that the checkbox has managed
to re-check itself without my knowledge or permission. At other times the
check/uncheck works as designed.

Any ideas on what is causing this? It's an annoyance, not a disaster.

Bob Gardner