how many miles has it done?

those boxes tended to have a life span of about 80k miles before they were on pot luck if they survived any more or not

might be worth getting it to a gearbox place to get the fluid changed and see if things get any better, shouldn't cost you more than £100 all in and at least you will know what the problem is

if the gearbox is fubar then it's probably time to invest in a new car if i'm honest unles you've ploughed lots of money into this one