Of course I can't really tell you what causes your nub to malfunction, but here's my experience with nubs:
I own one of the early Pandoras which are known to have flawed nubs, which do not last that long. After some times during which I didn't use the nubs that much, things got so worse that i had the same issues like you. Out of some explorer-mood I decided to try to change the nubs on my own. Two of the new nub-generation. Exchanging them was not that easy and due to my low experience with stuff like this, I damaged my board a little. I can't really recommend to try changing them on your own unless you have quite some soldering-experience and/or can stand losses.
Still I managed to get the new nubs working. And they worked fine. Now after some time, I am having the same problems again. Again without using the nubs that much. Most of the time I use shortkeys or synergy/ssh (an external keyboard) to control xfce. I have the slight feeling that the soldering contacts are not perfect, as I opened the Pandora again and applied some heat to the spots. And for a day or two things were better. Not sure if this was due to resoldering or a coincidence, but it didn't last long.
So today I will try to fix stuff again. We'll see with what result.
In short: Not sure if your nubs or the soldering spots (unlikely) are bugging you, but in any case I recommend to let someone fix it who knows what he is doing.