When accelerates the rpm jumpes and the car jerks then its

[alketraz]

I have a S40 2001 87000 miles and when I accerlerates heavily the rpm jums to higher levels and the car jerks, but then a second after that it comes back to normal, this does not happen when I accelerates normaly, I have heard that there is a sensor around the transmission that needs to be replaced, not sure if thats the cause and what is that sensor?.

Thanks a lot.

Del