Probably a positional detent in the shifter relative to brake pedal/steering wheel position. If you can see that little tab in the corner of the cover plate on the shifter that looks like it's covering something (usually the size of a nickle), pry that sucker up with a flathead (wrapped in electrical tape so you don't scrath the cover), unfasten the fastener under it, remove the cover plate and start cleaning with some electrical spray cleaner type something.
I mean, I reckon. Get the cover off and just have a look first.
Also that black, sectioned, sliding cover band that sits at the base of the shifter handle to cover the hole in the floor can get all gummed up, too, and cause all kinds of trouble.