I'm having a similar problem with my 99 S80 2.9. It use to start up really rough some times, didn't matter what temp. i live in texas so it's always 100F! It would run rough for a while and then go back to normal. Then it got worst, one day as i was driving i stoped at a stoplight and the engine started working really rough and the check engine light came on, when i pushed the pedal to go, the car would not go over 15mph! the text window said stop & restart cearfully so i did that and it didn't help. While I sat and waited for my dad to come get me i started the car and it was perfect! There was a code saved in the computer for the check engine light, PO121, pedal position sensor. I went to the dealer and got a new accelerator pedal, the sensor dosen't come seperate and i just changed it and it ran fine for 3 hours and it did it again!!! Right now my dad is messing with it, i don't know if this problem is related or the same as yours but i'll keep everyone posted about this issue. If anyone else knows anything please email me at veso_ht@abv.bg.
Thanks, Veso