It could be several things but start by finding out if you have 12 volts at the starter "s" wire when it doesn't start. That's the small single wire the just unplugs (female spade connector) from the starter solenoid. If you have 12 volts there when the starter's not working, then the problem is within the starter itself and I'd recommend replacing it with an OEM Volvo starter, not aftermarket.

If you don't have 12 volts when it's failing, it could then be several other electrical items but before I list all that, check the wire above first. That also "assumes" that your battery posts are clean, tight and that the battery cables are also correctly tightened as well. I've seen loose battery cable ends (engine block near trans in front) do strange things too and often a starter is misdiagnosed due to a bad ground connection, battery cable end at block.