Thanks everyone. I flushed the heater core again tonight. Not much debris, at least not enough to cause a significant clog. I dropped all the air out of the rear bags and inflated the fronts all the way. I also jacked up the front as high as I could. I was able to squeeze another half gallon or so of 50/50 into the radiator.
I think my issue may be a compound of a few issues. I think my engine runs on the cool side, especially when it's moving with air flowing through it (see picture of my temp gauge below). I noticed with a piece of cardboard blocking the radiator that the gauge moved further than I have ever seen it, maybe to 1/3 the way between C and H (assuming the picture shows around 1/4).
