Gearbox slip - Advice?

[fast cars]

I have a 2001 E320 Diesel saloon and it seems to slip when changing between 3rd and 4th gear, this is shown by a raise of rpm for approx 1-2 seconds and the rpm rises approx 300rpm between the gear-change. Its as regular as clockwork and not much affected by hard or gentle acceleration. Can anyone out there give me some advice on a fix??

Thanks
Danny

[Pontoneer]

First thing is to check fluid level - change ATF & filter if not been done recently ( last 50,000 miles ) .

[fast cars]

Thanks, will give this a go.