Photoshop brush mystery... any clue why this brush misbehaves?

Here's a photoshop brush mystery that I have spent hours trying to figure out... without success.

Maybe someone here can tell me what I'm missing?

I have a brush that I've made up in Photoshop which consists of horizontal lines. 
This brush misbehaves without rhyme or reason.

Sometimes the brush will paint directional... that is to say the lines will change their orientation from horizontal.
In the past I have had this directional orientation occur when I click on the 'shape dynamics' in the Photoshop brush editing dropdown box.

BUT at other times when I have the exact same brush clicked on 'shape dynamics', the brush will NOT change direction... but always paints only in horizontal mode.

The linked flickr file will help explain this more clearly.
I have made up an example showing the brush with directional changes and fixed only on horizontal.

http://farm5.static.flickr.com/4122/4754966391_ebba0c7ecb_b.jpg

WHAT in blazes would be making the brush behave one way one day and a different way the next?
I spent an entire morning clicking and un-clicking every possible combination... and have not arrived at any conclusion.

THANKS for any help!

I'm on OS 10.3.9 running CS 1