I have found that my signature randomly disappears. The Signature box will be
greyed out, but I uncheck the "Use MS Word to edit e-mail messages" (Tools>
Options> Mail Format tab) then I can reselect my signature.

It seems that something re-checks MS Word to edit box every few days.

Does anyone have any idea where a setting for this is as, or what might be
causing this problem and how to fix it???

Thanks