4th gear problem

I just bought a 93 Z28 today with a 6spd.  I got it so cheap that I didn't bother looking at the flaws more then once.  Heres the problem.  Occassionally when I shift it into 4th the stick starts viberating in place as if it wasn't in it's proper spot.  So all I do is re-apply the clutch and pull it even harder to ensure it's in the gear fully and it works like a charm.  I at first thought it was my fault for not putting it in gear all the way but it doesn't do it in any other gear, and plus I shift kind of hard anyway so I don't see that it could be my fault.  This is my first manual transmission and I was wondering if this was a sign of 4th gear going out.  Can anyone help?

I have a similar problem, if you put the car in fourth, hold the clutch and rev the engine does it ever pop out of gear? I first noticed mine did this when I was in a tunnel and I wanted to hear the echo sound which by the way is really cool.