E46 m3 not always going into fear properly

Hi, I own a 2002 m3 Manual which I sometimes have trouble with the gears.
Sometimes when you try and select reverse the gear stick appears to go in correctly but as you release the clutch the gear box grinds and the stick jumps back, when I try and reselect the gear it crunches in also it crunches into gear a you change down gears into corners etc
Does any1 have any advice?
Do you think it's the box or just the linkages?
Any advice welcome thanks