Vrs Rev Counter drops when accelerating @ 4000rpm

Hello,

Searched forum and found similar but not the same.

When accelerating in 1st - no problem.

In other gears, when accelerating, the rev counter hits 4000rpm then drops to about 3000rpm then climbs again. It does this in gears 2-5. I'm not feeling any loss in power or none that I can notice (think any I do feel is psychological).

Last motorway journey I was doing 65 at ~2500rpm, put foot on the floor and the car accelerated fine only the rev counter climbed, then suddenly dropped then climbed again.

Not sure if this is clutch slip or a faulty sensor but wondered if anyone had experience the same. It's only slipping the once each acceleration (i.e. will slip once then rev fine to 6-7000rpm) and it's not every time and only when pushing it hard.

Will have to take to garage (due a service in 4 weeks anyway) but without them going out on a dual carriageway / motorway, they'd be hard pushed to replicate on a 30mph road.

Thank in advance,

Damo