Heater element tripping GFCI

I'm trying to get a sauna heater running in a new sauna we have here and am having problems. I have two old heaters which are both showing similar properties we'll call it for now (maybe problems once it's clarified)... The first heater ran for a few minutes then tripped the GFCI. The GFCI wouldn't rearm after this (instantly trips). I pulled it apart and tested continuity around the two elements, and continuity to ground, finding there was a varying resistance of 10K-150KΩ ohm between the element terminal and ground. So I decided this was the source of current leakage, faulty element, onto heater number 2. I wired heater number 2 to test it and it roasted nicely. I got to work cleaning it up, sprayed the element and grill with water to clean and then properly installed it. Upon powering up this time it tripped the GFCI. So I checked the elements of this heater. I'm finding a similar result. Of the two elements, the top one isn't so bad, with the multimeter set to 200K I get a 150KΩ+ reading for just a moment, then open. The lower element is all over the place. From the element to ground, it starts at 10KΩ and climbs over 200KΩ over several minutes. If I disconnect the probes and touch again it resumes from a little lower than where it left off. If you reverse the probes it starts again from ~10K and climbs. Almost behaving like a non-polar capacitor. What exactly is going on here...? (Also - I have ran the heater with only the top element connected to the hot wires and it worked for an hour + without tripping the GFCI, until I turned it off. I was hoping the heat would dry it all out and resolve the problem. I tried the lower element alone and it trips the GFCI even after an hour of the top element less than 2" away baking.)