Mk3 who knows anything about transmission?

Currently reverse gear works fine but putting it in 2 the gear makes a grind and a loud clunk noise that jolts the whole car!
1st gear is usually fine until after about 10 mins of driving when it starts to become difficult and sometimes impossible to put in to 1st!
this can be embarresing at a red light turning green! 2nd 3rd and the rest r fine!
The clutch is starting to slip a bit but i dont see how that would affect 1st and reverse gear!

any ideas?

slipping clutch does effect it..