Hi Guys,
Been starting to convert an Ultimaker2 (2nd hand) machine to Duet with a Duet 3 Mainboard 6HC mainboard including the PT100 extention board for PT100 compatibility.

Board: Duet 3 MB6HC (MB6HC)
Firmware: RepRapFirmware for Duet 3 MB6HC 3.4.5 (2022-11-30) - didnt bother to update yet.

So far the wiring looks fine - basic functionality is given.
Unfortunately when heating up the printbed the values start going up as intened only in the beginning (from around 22- ~45 °C), then the reading starts spiking.
When restarting the machine/board the reading looks fine again.

Any pointers what the procedure of debugging would be here?

; Heaters
M308 S0 P"spi.cs0" Y"rtd-max31865"         	; configure sensor 0 as PT100 on pin spi.cs0
M950 H0 C"out0" T0                         	; create bed heater output on out0 and map it to sensor 0
M307 H0 R0.270 K0.221:0.000 D0.53 E1.35 S1.00 B0                           ; disable bang-bang mode for the bed heater and set PWM limit
M140 H0                                    	; map heated bed to heater 0
M143 H0 S130                               	; set temperature limit for heater 0 to 130C
M301 H0 P124.55 I23.46 D165.29				; old PID-Values UM (M)

M308 S1 P"spi.cs1" Y"rtd-max31865"         ; configure sensor 1 as PT100 on pin spi.cs1
M950 H1 C"out1" T1                         ; create nozzle heater output on out1 and map it to sensor 1
M307 H1 R2.103 K0.294:0.000 D8.88 E1.35 S1.00 B0 V24.0                           ; disable bang-bang mode for heater  and set PWM limit
M143 H1 S350                               ; set temperature limit for heater 1 to 280C

After restarting the board