In Sketchbook Express, it appears as though I also experience some of the ‘wavy-line syndrome’. On my Modbook, unlike yours, it appears to have no particular preference for diagonal lines:
This is undoubtedly radio frequencies generated by internal components of the tablet interfering with the digitizer's detection of the stylus. That the problem is much worse in the upper left corner where RF interference is strongest confirms this conclusion:
Note that my Modbook was one of the very early models, and it never received the additional shielding associated with the Bluetooth update, so I'd expect this sort of problem on my Modbook to be worse than most.
You may want to contact the vendor who built your Modbook to see if they'll install extra shielding.