Heated bed keeps getting hotter while printing (no replies)

Hi!

For some reason, since a few days I have a big issue with my RAMPS board. I don't know why, but when I print, after a while, the heated bed becomes hotter and hotter. At first, setpoint is reached nicely, and things settle down. However, after a while, the readings get hotter, and keep increasing. I once reached 110degC with my setpoint at 60degC. Obviously, I shut things down, as I do not trust this behaviour.

I tried multiple prints, and all now have this issue. The thermistor seems to be working, as it is reading the temp, and indeed the heated bed gets abnormally hot. What I did notice a while back is that for some reason, the readings of my thermistors (bot bed as nozzle) all are quite jitterish when temperatures are low (ie heating is shut off).

These are the readings I get while printing:
[attachment 53576 print-temp-readings.jpg]

1. Start 1st print (this is a larger print)
2. Without any interaction of myself, the bed temp starts to increase (setpoint did not change)
3. I shut off printing, and put on a new 3D model
4. I started printing the 2nd print (smaller different print)
5. Temperature again starts rising at about layer 3. Temp rise is much faster, maybe because it's a smaller print.
6. I shut off the printing, and all heating. As you can see, for some reason the temp readings are very jitterish.

This is a detail of the last minutes when the temperature control was shut off:
[attachment 53577 temp-detail.jpg]

Could it be that the jitterish readings are messing with the PID controller? What might be the reason for this malfunction?