My 98 DiscoTdi manual, has at times been difficult in changing gears, as though they aren't in the same position, and also when driving in say third with your hand on the gear stick, I have noticed that it can get a bounce to it. Just doesn't feel as tight as it used to be.

 

You need a new clutch.

 

How're the clutch hydraulics? Tried bleeding it? The baulk rings could be worn excessively.
Also you shouldn't rest your hand on the gear selector when driving as it causes increased wear in the gearbox.