If I am understanding what you are trying to say, it sounds like the transmission is slipping and allowing the engine to rev higher than what you'd expect for the speed you are going. Not knowing if it is an automatic or standard gearbox, I'm going to assume it is auto, because I don't think Volvo made too many standard shift 4-doors.
Something you need to check is the transmission fluid. First, while everything is warm, check the level of the fluid in the transmission. While you are doing this, check the color and smell. You want an oil smell with clear red fluid. If you smell a burnt smell and the fluid looks brown. You can try to change the fluid to see if it helps, but this is usually just a temporary fix. Once the fluid is burnt, it usually only a matter of time before the transmission is done.
If the fluid seems good, it doesn't mean there isn't a problem with the tranny. You may want to run it by a transmission shop and see what they think. Don't forget to get a second opinion with a mechanic you trust.
If you check the tranny and it seems okay, you'll have to post up more information about what is going on so we can give you better advice.
EDIT: With the new knowledge of a standard transmission, I would suggest the clutch is slipping as suggested.