Validating your oven temp can be a difficult challenge ...
I've been seeing quite some swings on my thermistor measurements (this is just for validation, not part of the control loop) in the order of several tens of mK, at the same time my ADC measurements have typically been within +/- 4mK ... so I've just put in a four wire PT1000 as an additional measurement.
During the same 30 minute period I saw the following results...
This is the PT1000 (measured on a DMM7510) in four wire mode...
This is a 5k thermistor (measured on a 34470A)...
Quite mad ... there's presumably some level of equilibrium going on (with the PT1000, which was new into the device) but the reality is that the temperature should have reduced (because I removed the lid to put the PT1000 in) and then should have been increasing again one the lid was back on.
So I believe the PT1000 at this point, but I'm going to continue watching both of them for a while!
EDIT: Err ... user error. Both devices weren't particularly well fixed and the oil level was slightly low so I think I was getting a bit of a hybrid oil/air temp. Now that they are better fixed they actually match each other reasonably well ... I'll still keen an eye on them for a while.