When I start my S60, it runs extremely rough and will eventually stall. I do this over and over . . try and put it in gear and it will go a bit, run very rough, and will stall out. After a few minutes, something kicks in and it runs perfectly. If I don’t try to drive, and sit with it idling, it runs very rough and I have a difficult time getting the rpms up. Then, the engine will suddenly race for a bit and everything is fine. It is very periodic in nature, doesn’t matter if the engine is cold or not. It can sit over night and be fine, or do it, I can run around all day without problems and then suddenly it decides not to start up and run. The temperature outside doesn’t seem to be a factor.
Is the check engine light on? If so get the codes read and report back. If not I would have your fuel pressure checked. You could have a flakey coil or a bad injector, but I would start with fuel pressure.
You may have a problem with the ETM (electronic throttle module) that apparently plagues Volvos of that vintage: