Strange behavior from the bed temperature  

Strange behavior from the bed temperature

Hi everyone-

I've had my MK2 for years now, using it mostly in spurts, and almost exclusively with PLA.  I've recently started using PETG, and have been having an occasional issue with the bed temp.  When going above around 80C, it seems that the thermistor just doesn't read correctly, and I get a bed thermal runaway error.

What I've seen goes something like this.  Using the generic PETG profile in prusa slicer, the bed temp post first layer is 90C.  What I see is that the bed warms up to the first layer temp of 85C, and prints the first layer correctly.  The bed then tries to rise to 90C, but something goes wrong.  I see the bed power LED on constantly, and I see the actual temperature of the bed (measured with IR thermometer) rise, but the prusa displays temperatures like 83/90C.  Since it thinks the temp is low, it keeps trying to raise the temp, and I see the actual measured bed temp get to around 110C or so before the system shuts down with a bed thermal runaway error.

Now this doesn't always happen, and seems to mostly happen only with small objects.  Any ideas?  Is the thermistor bad?  I'm not sure why it would only be a problem with higher temperatures.  I did try the bed temperature calibration routine, and it worked fine until 90C, when it got stuck at 89C, despite the bed temp increasing.