What causes a linear taper slide pot to develop dead spots?
Dead spots in pots are generally caused by buildup of less conductive contaminants on the surfaces contacted by the slider.  They can often be removed with a red pencil eraser, although disassembly and reassembly may be more difficult than the removal itself.  In this case, it's also possible the contacting surface has been eroded, since this is the range the pot is used in the vast majority of the time.
From your description, it sounds like you're seeing some hysteresis between two settings.  This could be a different phenomenon, or it could be an artifact of the other circuitry.
Even though the pot itself is discontinued, it may be possible to find a replacement if you can find the specifications of the part.  Of particular importance is the power dissipation, which could require specialty parts if over 1W.