x11 not properly set

With my recently installed opensuse 11.1 on my hp tc4200 tablet. (laptop), , X11 is not working well: sometimes windows disappear for few seconds and other minor weird things happen.
My graphics card is intel 915GM

Inspecting the /var/log/Xorg.0.org, i have found three lines with the error message

(EE) intel(0): underrun on pipe B!

What is its meaning?
Moreover:

usbDetect: can not ioctl version
Wacom xf86WcmWrite error : Input/output error

Any hints?

Thank you
---pol