Help with temperature detection

Hi there,
I am having issues with my 3d printers temperature detection. It's an Arduino and RAMPS running marlin with an e3d v6.
The temperature readings seem stable but can suddenly jump ~20-30?. I initially thought this was a connection problem of the thermistor so I checked the cables and didn't find any problems. This behaviour also happens whilst the effector is stationary, so the cables are stationary and so there shouldn't be any changes in connection quality causing different temperatures.
I also noticed that when I reset the printer, the temperature reading often swaps back to its previous value. Earlier today, I plugged in an SD card into the printer and the temperature immediately dropped ~30?. Then when removing the card, the temperature jumped straight back up. This seems very strange to me.

Any help would be much appreciated!
Thanks,
Tom.

Not sure it is applicable for your unit but there is a PID calibration I did on my machine (makergear M2 Marlin ???) Set temp to 150C let stabilize, send 303 and wait till it is done and spits out the final number then send M301 and what it tells you at the end, then a M500..
Here is an example of config setting for one of my hot ends
Do not use this numbers as they are specific to the hot end it was run on
M301 P18.64 I1.24 D71.83 then 500

I switch hot ends to switch nozzles as it is faster.. Each hot end is slightly different so I have those setting in a file and just set them as part of the swap process
Good luck

Thanks for your reply.

I have used PID autotune in the past, however I don't think this is a PID loop problem as the temperature detected changes instantaneously by a large amount.
I think I will try manually measuring the resistance of the thermistor and try to make the printer read different temperatures to see if it is a problem with the thermistor or cables.
Could it be possible that my Arduino is measuring voltage wrong?

Thanks,
Tom