This doesn't really sound like a monovalve problem to me. Why would a bad monovalve provide heat at idle, but not when driving? I guess it could be sticking closed sometimes, but that sounds unlikely. One way to find out would be to unplug the monovalve and verify that you get maximum heat. If you get max heat with it unplugged it may be the control unit acting up. I would give that a try before ordering any parts.