1999 SL500 transmission fault code help
 

Hi everyone, this is my first post.
I've just bought a 1999 SL500 with 55,000km on the clock.
It drives fine but when you put foot down up a hill it's quite a rough kick down then kick into another gear.
The fault code that comes up is 146 - RPM n3 too high external tooth disc carrier has anybody got any idea of what this means?
It has a full service history at mercedes and had it's major service at 51,000km less than a year ago with nothing else noted that needed to be done.
Any light shed on this would be greatly appreciated!