1st and 2nd gear change problem???help

[gypsyboy]

hello folks we have a golf tdi in the family its a 1.9 1997 mk3 it has covered in excess of 150.000 miles and is still sweet.

right the gear problem the gears will be good for 3 or 4 or 5 weeks then all of a sudden there is a problem with 1st and 2nd when you go for 1st it slides in to 3 and you really have to give it some gyp to get it in to 1st and similar with 2nd it is difficult to engage and sometimes slides into 2nd and even trying reverse you go in to 2nd????

then all of a sudden after upto a week it goes back to normal.

so any ideas please if you have managed to read all this