[Maxwell]

With the helm pulled forward, shift in and out of gear while looking for anything on the underside of the shift mechanism binding. Make sure all of the u bolts holding the cable end in place are tight... If you don't see anything, disconnect the cable and see how the shifter feels

Are the cables original? If so (actually either way) I would triple check that its not the cable. The usually give you some warning by being harder to move etc before they fail. They usually tend to fail right as you're trying to maneuver in tight areas and it gets a little exciting... I learned that one last summer thankfully without incident. Either way, the cables themselves are not that difficult to replace.