I have a 99 SE 5spd. About every other time I shift into fourth gear the shift is a little rough. The car only has 15,000 miles so I don't think it is wear. Any possible causes? Thanks.

 

Hard shifting could be caused by wear or a defect in the synchronizer mechanism. It could also be caused by a dragging clutch. Verify that the clutch master cylinder fluid reservior is full. Make a conscious effort to press the clutch pedal all the down when you change gears.

Some Maxima owners have reported that synthetic gear lubricant improved their car's shifting feel. I have no personal experience with this; my Maxima tranny is still running on the factory fill.