Possible thermos valve sticking - Bar's Cooling System Lube?

My car keeps throwing a code for the thermos on the cooling system. The causes for the code are sticking valve, defective pump, or defective sensor. (When I clear the code, it often does not return for a few weeks. It has also turned itself off a couple of times.) Before the check engine light illuminates, I usually HEAR the pump run louder than normal after engine shutdown, as though it were running against a closed valve. So, I obviously suspect that the valve is sticking intermittently.
Dealership refuses to give me an estimate, except to tell me that evaluating the problem will be $125, and that I cannot rely on the reading of the code by anyone other than a dealership service department. Oh, and a round trip to any dealer is 150 miles. My '09 Prius has been trouble free until I recently took it in for two recalls; main water pump and steering. The recall was NOT for the water pump in question, but of course the coolant was removed. My second suspicion is air in the line, which could make the pump cavitate with a similar noise.
So I found this product, "Bar's/Water pump lube and anti-rust" (at AutoZone) which I'd like to try. The label claims compatibility with all coolants, etc, etc. However I'm not hot on the idea of using unknown additives.
Is there ANYONE out there that has used this stuff or encountered this problem? I'm open to all advice! Thanks.