I've been experiencing the same issue, but largely when it's raining or snowing, or otherwise damp or humid out. I'm wondering if there's some sort of an electronic shift module that might have a weak or torn seal, and that there's some sort of a short in it? Like I say, it only happens when it's wet out, but it's not just 2-3rd gear, it's all the way up the spectrum, and in and out of reverse as well.

I'll post if I manage to solve it. It used to be only occasionally when it was particularly wet or rainy, but now it seems to be consistently whenever the weather turns wet.

I've replaced the fluid twice during which this was an issue, to no effect, being very careful of level, and added the Lucas anti slip the last time.