Thank you so much for this, you're absolutely right on all these things. It just sucks figuring out which it would have been. There's been a revelation though, my heater core hoses had slits I didn't know about and today the hose blew, had to drive home while it was on the hotter side of the gauge. I've now bypassed the heater core using a single heater core hose from AutoZone. It's idling so much nicer after adding/bleeding new coolant, and I also came to find out I was low on power steering fluid as well. I think I'm on the right track :D