Thanks for the input, but I'm starting to think my flushing didn't work well from the start. In an effort to improve flow through the motor I removed my thermostat, removed it's guts, then reinstalled. I performed all of the procedures with this setup and felt like I was accomplishing something. Even though I couldn't get the engine up to temperature, I didn't think it would matter with the thermostat effectively open. I had the heater at the max setting the entire time. Today, I had to take a long drive (300 mi.) in which the temp got above 80C for a good bit of the trip. When I drained the system this evening the fluid was a deep amber - almost a muddy orange. Anyway, I installed a new thermostat and will continue to flush until things clear up a bit.

I've read the thermostat needs to be "forced open" to do a proper flush. I thought I was accomplishing the same thing by taking out the innards; however, maybe doing so allows too much flow, resulting in the engine not getting up to temp and coolant not being pumped into the heater lines. Does this make sense?

As for the citric acid, I purchased it packaged as "sour salt" at a kosher grocery store in Baltimore. It was $1.89 for 5.5 oz.