MQ350 PNN

Hi everyone.

I have a minor issue with my gearbox wondered if anyone has had anything similar with the mk3 octavia or a mk7 gti. So anyway on to it. 

Last month driving onto the motorway as you do going through the gears, got to 5th gear and it just ground. It stayed this way for the month tried it now and again and it just grinds. So I just went from 4th to 6th and it was fine. Well on my way to work yesturday went to put it in 6th and 6th now grinds but 5th works!!!! Annoying. 

So I've been searching for low mile boxes and found one but before I swap the trans could it be the cables? Should I realign the gear stick and box first.