Starving for fuel? Priming pump?

[SD Blue]

On my '84 300SD, 350K+ miles, I seem to be having a fuel problem.

The IP sounds as if it is not getting enough fuel? After running a while, esp. in hot weather, the IP sounds like it is running dry when I rev the engine and has a "chirping" sound at speeds above 60. Originally, there was a defective hose to the alda which I have since cured. But this seems to keep reoccuring when it is hot.

I have pulled the fuel screen and found it sparkling clean. The fuel filters are changed nearly every other month ( I do a lot of daily driving - service work ).

I have tried to see if the fuel tank vent line may be obstructed, however, I have not located the "end" of the vent line. When I pulled the trunk divider, I can see where the vent hard line goes through the floor. Looking underneath, there is a flex line with some sort of in-line "filter", then a hard line running forward. Does anyone know where this leads? I am beginning to doubt this is the problem because I test drove it with the filler cap off.

I have a feeling that the priming pump is going. Changing the hand priming pump had no effect. I would hate to lay out nearly $200 for a new priming pump and find out that it may be a free - $10 fix.

Are there any other possibilities? Maybe in the fuel return system?Would weak fuel return lines from the injectors cause this?