Gear slipping?

[evansa]

Hi, any advice appreciated I have a 18plate t6 kombi 2l diesel tdi 150. It generally drives great and had a new gear box and clutch last year. We've noticed that occasionally and only up hill (may just be a coincidence) that going from gear 3 to 4 it over revs and seems to slip/delay going into gear but then carries on fine. It doesn't do it all the time it just randomly does it occasionally, any ideas? Ta