Hello I was here when I was experiencing the very same problem wih my Volvo V50 1.8 so here is what I did to solve the problem:
Pre requisite check ups:
1) Check that the air intake hose has no holes and that the filter is clean and fitted properly.
2) Check that throttle body position is not erratic (there must not be noise on the signal when it is idling or held at the same rpm level). (I used a cheap obd2 diagnostic tool with bluetooth via my android phone to confirm this)
The problem (At least for my V50)
3)Change the MAP Sensor. (Or clean it but it can be hard to get to it and its cheap so I would just replace it with a new one). This solved my problem. The car still behaved strange on the drive directly after the replacement but I haven't had the problem ever since. The vehicle usually reported "Throttle body stuck open" error but the problem was a dirty MAP sensor.