Gearchange problem

[Malvolio]

Hello there!

Just bought a Peugeot 406 2.0, which seems like a lovely car. However, I've noticed that when I change up from 4th to 5th gear under reasonably hard acceleration, there seems to be a quite pronounced vibration/shaking coming from the engine. It doesn't happen in any other gear, and it doesn't happen when changing down. Any ideas anyone?

Thanks!