I'll second that. (The part about the fluid being low, not that the car is hanging upside down on the lower hemisphere.)

These cars are sensitive to fluid level. My wife's S-80 (why always the "wife's" car?) was doing the same sort of thing. I noticed, though, that it was only doing it when turning left, so I immediately considered the resevoir level. Looking inside it didn't "appear" low, but I figured what the heck and added a little Volvo pentosin based fluid. Bingo! No more throbbing of the steering wheel.

BTW - I also noticed the hose clamps on the resevoir like to get loose over time. You might check that to see if it needs an extra twist to snug it up. Or whenever you do T-belt service and have to swing the resevoir out of the way, check the hose clamps when you put the resevoir back in place.