You could try bleeding the system first.
Check out tech info at pelicanparts.com
I had a similar problem. Combination of things. Firstly there was a small leak in the radiator. Practically invisible to the human eye! No visible evidence of leaks on the floor. Only got picked up when system was pressure tested. Also thermostat was on the bung which didn't help!
As I was replacing the thermostat I drained the system and flushed it out. Before installing the new stat, I got some RadWeld and put that in the system and ran it per instructions. I then drained the system again, replaced the stat and filled with coolant. Ran it now for a couple of hundred miles and water level steady.