Pauls E61

Hi All
im new to the site and hoping I can get some advice

This evening I had to accelerate hard when joining the motorway

i lost a lot of drive, accelerating through the gears they seemed to be changing at the correct revs, but the car wasn’t going any faster.

Seems I’m ok on a flat road, accelerating up to 45, then it tails off and no more acceleration power/drive. Took a few minutes to get up to 80
normally when I boot it, it almost throws you into the back of the seat. Now it’s like trying to drive an elephant
its like I have lost half of the horse power

I parked on a hill at the shop. Pulled away and could only manage 20mph at 4500 rpm

its like how a manual behaves when the clutch is slipping

if you have any ideas on what the cause could be it would be appreciated
or is it a case for the auto gearbox specialists

thanks in advance