Well, it sounds like what I should do is:
1.) Apply barnacle buster and flush.
2.) Pull the wee plugs and dig out remaining rust, which is probably there.
3.) Change thermostat.
On the thermostat.. I do believe I put the old thermostat back in thinking, "they always break open don't they?"
I also do remember my temperature gauge going down quite rapidly a couple of times and then gradually rising back up (Sunday).
Does that sound like a plan?