Hi I'm new here I looked thru the post but didn't see anything that was related to the problem we're having with my 2001 Toyota Corolla. The heater wasn't blowing any hot air. We replaced termostat,, no change. We backflushed heatercore, at first it started to blow hot air but then the gauge showed overheating. The heater started blowing cold air again. So flushed the entire cooling system, several times now, water is running clear, but heater will work occasionally, gauge will go from hot to normal up and down no matter if I use the heater, A/C, or nothing. Any ideas what it could be. Thanks for any help in advance .