CountryCat, my bet is it's just getting old and tired. I've cleaned mine several times not only with vinegar and water, but also with Iron Out (we have a *lot* of minerals in our water). It still gets plenty hot; I just don't use the steam feature much anymore because of the calcium deposits in the holes.
Edit: Over the years I have acquired 2 backup Rowentas at Goodwill in case mine ever goes out. Each of the backups cost around $5, I think.