It could be too many things to list. In my experience it has been wiring issues, especially the datalink between different modules on the truck. But I've also seen it with missing fuses.(drivers trying to disable certain features. It won't be intermittent like yours). It would be best to find a qualified tech with access to Volvo Premium Tech Tool, schematics and Fleet Impact to diagnose this one. Throwing parts at it is going to get expensive, and frustrating. You will probably save money in the long run to take it to a shop that is familiar with Volvos. And as much as everybody hates dealers, they're usually the best suited for these things. Good luck friend!
Nothing you have to do most of the time this comes on is because battery connection loose or truck lost battery power could also be kill switch on top of batteries make sure batteries and terminals clean and tight.