I don't have photoshop or sketchbook but the phenomena is clearly existing in Onenote or Windows Journal. The "wavyness" depends on the speed when you draw the line. Similar experience I have while writing - the text seems to be more smooth when writing fast.
BTW: I had (or still have) WACOM technology using devices before and there I've had a similar issue also with wavy lines but there related to the pen pressure (altering pen pressure at the beginning or end of a line/character). Slow drawing 45° lines no problem, however and as mostly known, terrible accuracy on the borders of the screen.
The n-trig digitizer does this with slow strokes, MS and m-trig are working on an updated driver that is supposed to include the ability to adjust the Pressure Curve and some additional settings targeting artists....no ETA as of now though