[v8volvo]

First thing: don't keep driving it after it gets hard to start and loses power. It may not need an IP rebuild now, but it will soon if you keep asking the IP to suck against a blocked supply line. Transfer pump cavitates, pulls chunks of metal off and pumps through the rest of the system, and pretty soon you are in for major IP work. With any VE pump diesel, it is critical to shut the motor down at the first hint of fuel flow restriction or else the pump will self-destruct in short order.

Good idea to identify whether the restriction is before or after the filter. As Anders suggested, check condition of all hoses and clamps. One neat trick with a primer bulb is that you can actually use it as a rough vacuum gauge for fuel system work. If you install the primer bulb in line before the fuel filter and get the car running, the bulb should stay full and open. If the bulb sucks down flat as the car is running, then it means the pump is having to pull hard on the fuel line to draw from the tank. You can do the same test between the pump and the filter, but it is more difficult since that section uses a molded line with banjo fittings, and you also run the risk of introducing contaminants into the fuel system that way... so I would just do it before the filter. If the filter is restricted, the only thing to do is replace it anyway.

Quick "primer" (heh heh) about fuel filters.... if you have a car that is repeatedly plugging them, it is a good idea to try to see what is getting into them. Take the filter off carefully so as to spill as little fuel as possible, then dump it out into a clean container. What came out? Any obvious crud, dirt, water, rust? If so, then there is going to be more where it came from and you will have to either keep replacing filters until it is worked out, or drop the tank and clean the system out. If no obvious contamination, then it gets more tricky as to what is causing the problem. Air ingress or, as you suspected, an occluded tank supply line is the most likely issue at that point.

Also, what kind of filters are you using? If you are using a cheapo Fram, Wix, NAPA, etc, those are much smaller in capacity and will plug 10x faster than a proper OE style Bosch or Mahle Knecht filter. The Bosch filters only cost about $8 so they are worth getting a few of if you are in doubt. The other advantage of the nice filters from Bosch is that they have the smaller inner O-ring secured in place, whereas the cheaper ones do not. The difference there is that, if that O-ring is not held in place, it likes to not come off with the old filter when the filter is removed. Then, when your new filter is installed, there is a double O-ring around the threaded snout on the filter head, which can cause the new filter to not seat all the way on the head and allow air to leak in past the outer gasket. Or, worse, if you forget to install that inner O-ring because it is not fixed in place, you can end up having a filter on there that is allowing some fuel to bypass unfiltered... not good.

So, short version: especially if you are going through filters, consider getting your hands on some better fuel filters, and take care when replacing filters to make sure that there is no old inner O-ring still stuck on the filter head after the filter is removed, and the new filter's inner O-ring is in place during installation. Since Volvos have the filter mounted down low on the block, it is hard to see or feel if there is an old O-ring up there... get a mirror or feel up there carefully with sterilized clean hands, and pull any old ones off. I once worked on a car that had two ancient O-rings piled up there! Needless to say it was hard to get it to run well until I discovered that. This became such a problem for Volvo techs back when these cars were newer and on the roads in larger numbers, that Volvo actually issued a service bulletin in the late 1980s instructing techs to check for it.

If none of this gets you anywhere, report back... there are other, more weird causes for trouble like this... plugged screen in fuel return banjo, reversed inlet/outlet banjo bolts, etc... but starting with the basics is more likely to yield a solution. Just keep in mind that the entire fuel system is under suction, so any air leaks are tricky to trace because they won't leave big signs of fuel dumping out anywhere. An attentive eye and sometimes ear is required to find air leaks in VE pump diesels.