By all means test your monovalve as suggested by CTaylor however if it tests good, the monovalve repair kit is your fix. Since your heat is intermittent however, I suspect a ruptured rubber diaphram on the monovalve piston. This seems to be the most common failure and is probable on your 20 year old car as it was on mine. That diaphram is part of the repair kit and to my knowledge is not available separately.