Jack:

1) Never run your car on anything less that 91 octane. I understand that this does not prove to be cheap, but the risk of engine knock is too great, especially in warm temperatures. The 850 does have a knock retard feature, but it is a backup device and is not intended as a daily used sensor for the car's operation.

2)Check the vacuum lines carefuly. Most importantly check the ones at the carbon canister. This is where the fuel tank vents to the 'atmposphere'. On a full tank there is little air in the tank so vacuum in the tank is minimized. When the tank is near empty vacuum draw is very large and must be equillized. Volvo's 850 uses a carbon canister to do this to reduce pollution. At low manifold vacuum this canister is cleared so the fumes can be burned. This may be why you experince your problem at low rpm's.

At higher relative manifold pressure, the canister is not cleared. To get your car (which I assume is an automatic) over 3500 rpm's you need mabout 1/2 throttle. This results in higher manifold pressure (relative Your car is always under vacuum...) and the fumes are not cleared explaining why the car runs well at this speed.

Keep in mind it may be the vacuum lines leaking, or the Canister clearing Valve may be blocked or plugged.

Hope this helps

Andrew P