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OS: Windows 7, 64 bit, 16bg ram, i7
Version: TVP Pro (11.0.09-64 bits)
Wacom: Cintiq Pro 13"

Pressing shift+drawing will almost always produce a little shift in the line position if drawing slowly. Drawing quickly doesn't make the problem
appear. Looks like a pretty big issue. See image for more info.
The bump is where I started drawing. I would press shift and then draw.

I know! I have the same problem too, I'm surprise they still haven't fix it! I have to use Lazy Lezumi Pro.

Try this: already start the drawing movement without letting the pen touch the surface, press shift, lower pen onto tablet.

If you've pressed shift before you move the pen, the software can't decide reliably where you want to go and judges from those small movements your pen does even when you think you're prefectly still. It's the same in Photoshop, although it works faster there.

I don't think what you said is the reason for the jagged lines, because as soon as one presses shift the software should be locked to
a direction it assumes, not change mid line. I believe there is a problem in the code.
I've never run in such an issue with Photoshop.