You haven't mentioned checking or replacing the fuel accumulator. It could easily be full of gunk and restricting fuel delivery at high demand to cause symptoms such as you describe. It should be located underneath, mounted next to the main fuel pump. In the fuel delivery system it's between the main fuel pump and fuel filter. It's purpose is to help smooth out pulsations in fuel pressure. It also acts as a backlflow preventer to maintain system pressure when the engine is off, also helping to prevent vapour locks.

It's a simple design, a fuel chamber reservoir with a spring loaded diaphragm that responds to fuel pressure. Any gunk around the diaphragm or fouling the large spring could cause fuel delivery issues, acting as a flow limiter. It's a sealed unit and generally not considered serviceable. You could try bypassing it for testing purposes if the output fitting matches the fuel pump, which as I vaguely recall from others it does.

Bosch p/n 0438170001 is for 1974-on 140/240 K-Jet and commonly available from Volvo aftermarket suppliers, also on places like Amazon. It's not overly cheap, so do shop around. RockAuto carries it and is one of the better sources. A number of other makes with K-Jet use the same one.

Going back to thinking about the fuel distributor. Some of them have a small filter screen insert in the fuel line ports, possibly just the intake port, I forget. That's another well known place for fuel flow restriction.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now