Citroen xsara picasso - Gearbox - Windtalker

We bought a citroen xsara picasso 2004 a month ago but a few weeks later 4th gear kept grinding so took it back. They changed the gearbox but now the gears have gone funny. Going 70mph in 5th gear and the engine is labouring like it's waiting to be put in a higher gear??? How is this possible

Citroen xsara picasso - Gearbox - Oli rag

Either the clutch is slipping or the replacement gearbox is from a different model with lower gearing.

To check for clutch slip, find a steep hill, select 2nd gear and from low revs boot it. If the revs increase much faster than the road speed, it's slipping.