why does gearshift get stuck in park intermittently?

Coffee, soda .. etc, tends to get spilled into shifter cavity and can cause *shifter lock solenoid* to stick .. 'Sometimes' spraying WD-40 into shifter cavity and shifting back and fourth with foot on brake will clean gummed up solenoid and allow proper shifting... WD-40 will not harm car interior, use it liberally on the shifter mechanism... There is also a manual 'shifter lock release' that can be used to by-pass the electric solenoid lock!