I have a 2011 6.4 hemi swapped into in a 69 truck, and a griffen radiator. with engine not running, and cool coolant. 100MV.
warm engine and coolant circulating. I get a radiator Eating reading of 650mv
I used a zinc anode it looks like new, i have added grounds and moved them around to different spots. still the same readings. i also removed the coolant and replaced it with RO water and royal purple PURPLE ICE......didnt help.
Sounds like you have a stray voltage if you are getting that much of a change in voltage while running. More going on than the fluid in the radiator.
I do not get a major difference in voltage. I'm around 80mV with less than 10mV change when running and hot.
I have a couple Justice Brothers products that might help, but they are not going to fix a bad ground. They are specifically made to reduce electrolysis with metal deviators to help with grounding between dissimilar metals.