If its in the cabin then the culprit is probably the heater core
Usually those products don't recommend being dumped into the overflow, you really go out on a limb with that stuff. I can't say anything because I've used Bars Liquid Aluminum(looks like aluminum flakes in the rad), to stop the crossover pipe o-rings from leaking. In the market for a job and can't be ripping the intake manifold off, I took the chance. If I get burned down the road I get burned, coolant is just green with little aluminum flakes. Glad its not a bunch of gunk, but that's beside the point.
like said above sounds like air in the system, but how is it getting there? Would a faulty thermostat cause wacky readings?