The first point of investigation is always get the codes read, not with a generic OBD code reader by by someone with ViDA & DiCE who will hook the diagnostics up to your car and read the DTC codes. They will not necessarilly tell you exactly what has caused the problem you compain about but they will stop you buying replacement parts and randomly thowing them on in a kill or cure effort. Also, you need to use various Volo enthusiast sites (use their search engine) to pull up posts from others who have experienced the same difficulties and how they solved them (if indeed theydid) I have been doing this for 20 years and still have a lot to learn.
Two such are
www.matthewsvolvosite.com/forums
www.volvoxc.com/forums
They have many more models of each in NW USA than here, and on the whole they seem more expert DIY mechanics.