It sounds like a fuel problem as opposed to an electrical fault. Start with the easiest fixes first ..
The next time the car comes to a grinding halt quickly remove the
petrol filler cap and listen for air rushing into the tank. If you can
hear air rushing into the tank that means a vacuum has been created
There are 2 causes of a vacuum inside the petrol tank. One is that the
air-valve in the filler cap is blocked (there's usually holes in the
filler cap covered by a rubber type ring) and the other is that the
breather tube from the tank to atmosphere becomes blocked with dirt and
debris thrown up by the roadwheels. Try driving the car with the fuel
cap removed or very loose so that air can easily enter the tank. If the
car still comes to a halt with the same symptoms, then there isn't a
vacuum being caused within the tank.
A failing fuel pump can cause the same problems. I once owned a VW that
had the symptoms you describe. The in-tank fuel pump was 'on its last
legs' and when replaced the car was perfect.
A blocked fuel filter can also cause the same problems.
Here in the UK the Volvo 960 is either a straight-six 2.5 litre or 3
litre engine. I would guess that Volvos in North America share a pretty
much the same mechanical layout depsite model differences. A UK 960 has
two fuel pumps. One is located within the fuel tank and is accessed via
an inspection plate behind the rear passenger seats (estate/station
wagon models). The other fuel pump sits in a bracket underneath the car
alongside the fuel filter, just aft of the front passenger seat (driver
seat in the US..).
The in-tank fuel pump does most of the work, lifting the fuel out of the tank. The under-car pump lifts the fuel to the engine (it shares the same fuel line). Start with the easiest fixes first - is there a vaccuum in the tank that 'slows'/'holds back' the fuel? Then look at the fuel filter which is supposed to be changed every 40,000 miles.
If it's of any use to you, I've put some photos of the fuel pumps and fuel filter on my own website - www.glennsmart.btinternet.co.uk
Just choose 'Volvo 960' from the drop-down menu.