Before replacing anything I would try to eliminate all possible non-fuel problems. If it seems strongly to be fuel related I would look into the fuel tank (in at least some of the V70/850s you can do this via the fuel pump cover). There may be dirt of some kind inside or the inlet screen may be damaged or collapsed somehow.

I had this experience with a much older Volvo. The tank was quite full of dirt and rust which sloshed around time to time and prevented the pump from taking fuel in and it was worst on a sustained upgrade.

If you can get a fuel pressure gauge it could be worth checking the pressure. On my older car the fuel pressure was low and fluctuated lots. The cure was replacing the fuel tank with a junked but clean(er) one.

I am guessing you do not have any trouble codes set - fuel pump or pressure problems wouldn't usually set them. Also I assume you have checked or changed the fuel filter, though a clogged filter would usually lead to a constant loss of power especially at higher speeds.

A hint for the operation of the fuel pump could be measuring the current drawn (could do this at the fuse block). I think that significant fluctuations of the current would indicate a poor pump or poor connections.