Wacom advises going to an earlier version of the driver, available on their web site, when you have intermittent pressure problems. Yours may be related.
Try version 6.1.5 from this page:
Ouch! That's no fun. Virtually everyone else has reported that driver version 6.1.5 has solved their pressure sensitivity problems.
I don't have a tablet myself, so it's hard to guide you with further detail. It's possible the downgrade didn't go well, or that some setting is off. I hope someone else can jump in here and offer further detailed guidance.
I figured it out! Pen Pressure is working just fine now. Here are the steps I used to fix my problem. Thanks for your help, Noel!
Windows 7/Vista Pro Driver
-Disconnect the tablet from the computer
-Open your start menu and then the Control Panel
-Open programs and features
-Uninstall any listing here for Tablet or Wacom Tablet and then restart the computer
-After restarting, download and install only the latest driver (6.16) from wacom website
-Once the installation is complete, and the system is stable, connect the tablet.