First, welcome to CHR! Hope you find what you're looking for and that you stick around for a while.

It sounds to me like you may have over-filled the radiator a bit if your still running an overflow tube to atmosphere. When you shut down the residual heat in the block & heads will continue to heat the stagnant coolant, increasing the pressure in the system and burping past the cap spring pressure. If I were you I would first install some type of coolant recovery tank, where the overflow tube hooks into the bottom of the tank, and the overflow from that tank is from a standpipe near the top. That way your system will burp when hot, but as it cools it will suck back the coolant from the tank. You can then run the radiator totally full, increasing coolant capacity circulating in the system, and eliminating air from the system which is the cause of corrosion.

Next I'd look at changing out that thermostat again, to at least a 180F if not a 190F. Running at 160F is too cold to cook off the acids that build up in your oil, and also hurts your mileage. All of the newer vehicles are running 195F or even 210F for normal operating temperature, to kick their efficiency up a notch.