Brush strokes become jagged after a few seconds

Trevor, not true!  You need to think about how mouse and stylus input data works to understand that, I'm pretty sure, this is a brush smoothing issue. It's not brush wobble.  It's not jitter.  In fact I think the program is working properly, but the notion of smoothing is kind of unintuitive so we don't always understand how important it is.

With physical pencil, as you try to draw a smooth line slowly, if the pencil tip moves very slightly, the mark will also do that in exactly the same amount.But with a stylus, a small movement may not register if you have not moved to the next screen pixel, or may register disproportionately if you were already close to the edge of that pixel where a very slight movement (less than a pixel) puts you over the edge.  So you move the equivalent of 1/4 of a pixel, but the stylus jumps a whole pixel, or you move almost a whole pixel (3/4) but the stylus doesn't move at all.  Anyone can see this quite easily for him/herself.   The stylus movement on screen is constrained to the (relatively coarse) grid of screen pixels.  This is why the effect is magnified when painting zoomed out. At 25% the above example would be you move 1/4 of a screen pixel, and the cursor jumps a whole (screen) pixel, which is actually 4 bitmap pixels.

So for example If you try to paint a vertical line, you will often see something that it looks like this:

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because you didn't move perfectly vertically, staying within one column of (screen) pixels.

How is PS to know whether you want this, or whether you are really trying to draw a smoothish vertical line?  I say smoothISH because I, for example, do not want a perfect vertical line, but I do want a smooth line, like what would happen with a real pencil or brush.  That's what smoothing takes care of, but in programs like Painter (where it's called "damping") you can set its level, whereas in PS it's just a check box. 

Damping and smoothing essentially round off the stylus input, so things like the stray bit of line in the above example don't appear.  The same thing happens when you use the shift key to make straight lines - PS only receives the first and last positions of the cursor, then connects them with a smooth line. This is the same as how painting very quickly creates a smoother line - there is simply less granularity, timewise, in the input.  I think that might, ironically, be why this "problem" does not appear right away. When you first start a program stuff is still loading and going on in the background and the stylus input may not have full granularity yet.

BTW, If I wanted to draw a perfect vertical line I would use the shift key, but that's not how I usually want my work to look. Sometimes I do and it's fine, but typically I want a more "freehand" but smooth quality.

Thank you all so much for giving ideas on this.  Honestly my best workaround is to rely heavily on "rotate view" so I can make marks more quickly and accurately.