440 overheats once and then it's happy all day

You're not buying the $3.00 "autozone special" thermostat for that nice ride are you? Those cheap ones are garbage. Ask for the $10.00 one. Much better spring and action. Also before you install, get a meat thermometer & place new stat in pan of water on stove. Stat should start to open @ the correct temp. THat will tell you the problem is either air in the heads or heater, bad temp sensor or gauge. Could also be a plugged radiator or collapsing bottom hose.

Whew. That should keep you busy.

Also could snag a cast iron pump & housing somewhere to try. 440 source pumps aren't very good.