What tablets are you all using? Is the eraser brush size and shape the same as set in Photoshop? Try selecting a square brush for the tools eraser, and seeing if you still have a square brush shape when the problem kicks in.
Have you got your tablet close to other electronic devices, like monitor or keyboard? Although my Intuos slots under my keyboard and I've not had any problems.
I see you have tried resting tools, and the tablet is not even plugged in. What about mouse drivers? Do you have a multi button mouse for instance?