85 Nissan PU 24Z loses power.

I recently started having intermittent problems with this truck. The truck will run fine for about 20 miles and the it gradually starts losing power to the point where floored I can only get 40mph out of it.

If I pull over and shut it down for a few minutes it will run fine again for another 20 mi then it loses power again.

I replaced the fuel filter which was pretty plugged but it didn’t help.

Any help would be appreciated. :)

consider cleaning the fuel tank out..