I totally agree with @Phaedrux , looks exactly like what I saw when a factory crimped connector on my RF100 went bad, started off slightly intermittent and I put up with it, then rapidly got to 'heater fault' levels.
I do note, as you do, how the graph flattens after the first layer. Maybe slice a model with the main temp and no fans on the first layer and see how it does. Mine was intermittent, but did occasionally pop up mid-print too. A failing thermistor might look the same too I expect.. start with the wires and if nothing seems bad, swap it out I guess.