Strange heating behavior

I'm having a strange heatup pattern on my printer. You can see the dip is a thermal runaway. PID autotuning results in thermal runaway too. My printer is all self source and my bed is self made so I have a cross drill into the center of the bed so the thermistor gets a reading of the actual bed plate temp. Not sure why it gets stuck at 85 but as soon as you reset and keep heating it follows about the same tangent of the first line. Does anyone have any idea of what could be causing this behavior?