Saw,
There are three things that I can think of that it could be. Do you have the check engine light? Does the car sound like its missing? 1. leaking intake manifold gasket, The way to diagnose is spray carberator cleaner onto the intake manifold. This would cause a change in Idle thus, diagnosing the problem. 2. Coil packs, Your car has six of them located under the "Volvo 24 Valve" cover. While the engine is running unplug one at a time. This will diagnose the cylinder that is missfiring, every time you unplug one the idle should become rougher, more shake to the engine. When you get the one that doesnt change that is the culprit. Turn the engine off, pull it out and swap it with anther coil pack. Turn the car on. If you experience an even more rough idle then its the coil pack. If you just have the rough idle then your prblem is the head gasket.
3. The head gasked is pretty easy to test. Run your car for a while then open the expansion tank of the radiator with the car running. Does the anti-freeze gush out as if there is too much pressure in the cooling system? look at your hoses do the look like they are swolen? This means it the head gasket.
If you have any questions please reply
Brandon