I went thru this last winter and it is a precursor to the pnp swithc failing completely. Shifting thru the gears with 15 or 20 times will sometimes clear the corrosion off the switch but will not cure the underlying cause. the switch is worn out and needs to be replaced. The car is going into a "limp home' mode, second or third gear but it is driveable. Bay 13 has an excellent article on how to do the replacement. I did it without the tool for alignment and it is fairly straight forward. I tore apart the old switch and it was definetly worn out on the contacts area. Seems like i remember getting a CEL from the pnp switch also.
good luck.