Fuel starvation isn't noticed at idle because so little fuel is used
then. It would be felt during acceleration and hill climbing when the
engine needs fuel to make power.
Poor idle when? At cold start or always?
The engine can lack power for several reasons:
Sudden onset of poor performance suggests bad fuel or gasoline mixed
with the diesel fuel. If that's the case you can drive it and so burn
the fuel or, if it's really bad, drain the fuel tank. Bad fuel would
account for sudden poor idle and poor power.
This old diesel engine has mechanically adjusted valves. If the problem
came on gradually and includes difficult cold starting the valves may
need to be adjusted to restore the engine's full compression.
Regrettably one also needs to consider that the engine may be badly worn
given these symptoms and mileage in excess of say 250K miles (400K km).
A compression test is the only definitive way to know if the engine
needs an overhaul or valve adjustment or neither.