Ok I have a non turbo 2000 s80.. I put a different motor in it and the motor before wouldn't do this but it won't rev over 2000rpm and sometimes barely 1500. I used the original intake and exhaust off the other motor, including coolant temperature sensor and thermostat, I've cleaned the throttle body and checked all the vacuum lines. What else could it be?? Like I said, the computer, intake, exhaust are all off the old motor and the old motor didn't do this

 

I had a similar issue with mine. This sounds like it is going into limp home mode where the engine does not go over 2000 RPMs and you can only go 20/30 MPH. It might be malfunctioning throttle. These throttles have a special plate in them that wears down over time. There was even a recall on them about a similar issue. I replaced mine with an aftermarket one from Canada and it has been great ever since.

Also you may have to make sure that the computer is talking to the engine. Because it is a different motor, you have to program the computer to recognize the engine. It seems like you have to do this with every electronic part on these cars. I would take it to a dealership and have them take a look at the computer and see if everything is talking to one another. If not, you may have to get a software adjustment, which is the cost of one hour of labor. ($150 for my dealership)

Personally, I would do the computer first because the throttle is $500 for the part alone, and it would be cheaper if you did the work yourself.

Hope this helps.