The pitch bend wheel, which is nothing more than a spring loaded variable resistor, has three wires running to it. If the center wire somehow became dislodged from the connector, then it would not work at all. It's very easy to get to once you open the case, which is held in place by about two dozen phillips head screws in the bottom of the keyboard. The keyboard opens front to back, somewhat like a book. The pitch bend and modulation wheels are attached to the same printed circuit board, but have separate connections.
As for the original problem, this could be the culprit if the connection is intermittently making contact.
Hope this helps,
PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!