When mine went in my 13 Taurus i didn't notice any fluctuating Temps. But was caught with loss of coolant happening only after I parked. My 2013 Ford f-150, it's pump failure was total loss of coolant and high temps immediately. Them blaming mods is b u l l s h I t. Thermostat opens on its own and is strictly mechanical. Fans only thing that can be tuned. My opinion is either system isn't burped or they've installed wrong thermostat. I use Reische thermostat in my SHO, no issues. And as a suggestion it wouldn't be idea to install a Reische in your car . Another problem is your impeller in the pump might be running intermittent from a defect or wear even though it's ran by the timing chain and going by an guess there is no coolant coming out of the wheep hole on pump yet. And if you have heat in your cab, your heater core shouldn't be the problem. I think your thermostat is the issue just because you mentioned fluctuating Temps, sounds like it keeps opening and closing when it shouldn't be.