I think the problem is in Q. Q are charging K with their values. I achieve good results when Q is preloaded with powers of delta_t.
See any article about wiener model. It has a good Q matrix.
A00 has to be multiple of delta_t power of 5
A11 delta_t power of 3
A22 delta_t power of 1
A01 multiple of delta_t power of 4
And so on
A resistance isn’t a particularly dynamic state, should be an unknown constant, but you might have a bin of resistors where they might vary. Taking one “randomly” out the bin makes it a random variable. The bin of resistors will have a mean value, so perhaps that mean constitutes a state variable. Perhaps someone starts putting resistors in the bin from a ...