Okay,

Street only XC70, 104K miles. When stopped at a light, I noticed the steering wheel was uber easy (like it was assisted) to move right/left while sitting still. Rotation of the steering wheel is about 5 degrees both ways when it is in this mode. The car drives flawlessly no odd tire ware, fresh alignment/tires.

No noises from pump. Do plan on flushing the perfectly clear PS fluid, (zero fluid loss). On rare occasion, turning the wheel while sitting still, there is a clunk noise and feel through the steering wheel. Plan on checking ball joints, tie rods, rack, rack mounts, column joints and wheel bearings.

Does anyone have a "standard" cause for this kind of issue before I really tear into it?

john

You might start with the steering shaft u-joints and other joints. The clunking feeling sounds like it could be a joint that needs lubrication. My 2002 Passat has an occasional “clunk” feel in heavy traffic when I am on/off the gas quickly. Believe it is one of the CV joints. Haven’t found it yet. Seem to remember my old 240 having a clunk in the steering before the lower steering shaft u-joint went south. My 850’s steering wouldn’t return to center correctly but after lubricating the lower u-joint it was fine.

The looseness may also simply be the steering parts loosening with age. At that mileage there very well might be nothing wrong.
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Will I buy another Volvo??? We'll see....

My 850 is also quite light. Roll the window down and look at the front wheel while moving the steering wheel. I have less than 1/2 inch of play before the wheel moves.
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Keeping it running is better than buying new

Check for something loose, you named it mostly, and don't forget the steering box and couplings.