When I leave the hotend on while not printing it stays at a constant temprature. Once I begin a print the temp drops from 195 to 170 and then thermal runaway triggers. I have tried using 210c and it also triggered at about the 3rd layer. I'm using Marlin with standard thermal runaway settings. Printing with 1.75mm white plastic using a j head style hotend.

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Prusa i3 Rework - Ramps 1.4 - E3d Lite6 - Full Graphic LCD Controller

Check your layer cooling fans, had a friend with this issue. The layer cooling fans were blowing air down, when the head got close to the bed the cold air would cool the nozzle so much that it triggered the runaway protection. We turned the layer cooling fans off and the problem went away.

Yeah I would first find the source of the drop and then maybe stretch or disable the runaway settings.