There've been lots of complaints with brand new fuel pumps that were bad right out of the box or shortly thereafter in the last few years, though if you are getting adequate fuel delivery under full load, but not at partial load, I doubt fuel pump issues.
Have you checked for a vacuum leak once the engine is hot? Sometimes stuff will be straight and tight when the engine is cold, but once hot and expanded, gaps will open up, allowing air in where air shouldn't be coming in. Carb base to intake manifold, intake manifold to engine block, etc. Especially on whichever carb is famously prone to warpage of its base (I don't have a carb, so I didn't keep track of which model carb has the warpage issue).
Vacuum leakage doesn't matter as much when you have huge volumes of air and huge volumes of fuel being used under heavy load, but when only a little air and a little fuel is being used under light load, a vacuum leak will have a much greater impact on the fuel to air ratio.
Does adjusting the choke (reducing the amount of air coming in) when it's acting up help any?
--Mark
1973 SIII 109 RHD 2.5NA Diesel
0-54mph in just under 11.5 minutes
(9.7 minutes now that she's a 3-door).