Hm, I know this is annoying on par with 'is it plugged in'... but have you checked your brush settings to make sure they are set to taper? And made sure your photoshop and tablet settings are correct?
I've found that some drivers do work better than others, so you may want to check for an older version of the driver. I use CS2 and remember at one point I had to install a really old driver to get the tablet to work the way I wanted it to.
The only other thing I can think of off the top of my head is that your pen may be malfunctioning. How old is it, and how is the nib?
Also you may want to try plugging it into a different port. If it's not getting enough power it may start to be funky.