Volvo knows the cause and so should your dealer. It's mostly because your engine is probably not driven at high enough rpms and perhaps also combined with petroleum motor oil, possibly not changed often enough.

If you're not using synthetic oil like Mobil 1 or Amsoil, it'd be highly recommended. Driving the car harder (higher rpms) should also help.

The valves burn because they're suppose to turn normally but need higher rpms to do that. If they don't turn, they stick in one place and burn. This has been a known issue with all earlier white engines for years but I havent seen it in years so apparently something's changed in design.

A reman cylinder head might also be something you should consider.