91 300SE heater had a mind of it's own. Run Ok then start blowing constant heat. Turned out to be the monovalve. Replaced with the kit. Worked fine for about a month, now it is acting up again. Same symptoms. Do monovalves fail that often? Is there something else I should be checking?

You may now have an issue with the inside temperature sensor. Maybe the small motor behind the glove box is working intermittantly. Do a search!