It sounds like you have one of the models that has given the 850 a bad name for maintenance expenses. We ('94 850, 140k miles) have been luckier, with only two significant problems: the heated seats and no hot air to the feet (the dealer did not solve this one -shame on them - which turned out to be a bad thermostat.

Our Volvo now consumes about a quart/2k, and since my wife is the primary driver, I have to be vigilant.

I presume you have replaced the fuel filter (located next to the right rear wheel at 60k per spec? If not that might account for the behavior. The no-load behavior suggests this to me, since not much fuel would be demanded then.

On other cars, bad spark plug wires have accounted for similar behavior.

I am not a mechanical expert, but have been around for more than a few years. Hope this helps.