could very well be your clutch on it's way out mate, i had exactly the same problem at first it would get stuck in 4th after giving it a blast it was a 1.4 then cure itself it got worse and i started having trouble with all the gears was fine when cold but after a bit of a run it would play up, is it biting pretty high, i'd take a guess your clutch is on it's way out tbh as it sounds just like what i had