Is the shifter in the floor console in your 1996 Chevrolet Lumina? If so is it possible coffee or soda has been spilled on the console and into the shifter? It is not uncommon to have shifter issues caused by the residue from spilled drinks. If this could be the case, removing the shifter and washing it under hot water and thoroughly drying it should remove the residue. If the shifter is not the problem then checking the shift cable would be the next step.