If you have poorly adjusted valves then that would be a CRITICAL diagnosis for ALL the symptoms you are experiencing. Go out and do that number one. Do the valve job, check and lube the throttle linkages ASAP. Then, take a look at any problems you might still be experiencing with the certainty of good vales and linkage.
Isolate the tank by making fitting a small "test tank" right in the engine bay. If your still having problems with that.. check the lift pump. Get the rebuild kit from peach parts or get a brand new one.
240D Lift Pump Work - YouTube
Fuel pressure was the root of the problems i was experiencing on my latest go round of benzolack wrenchin. Things i shoulda checked first i finally arrived at after chasing several other things, including the tank strainer.
Fuel pressure is pretty crucial for the ole 617. Sounds like you're on the right track. Go 'head and make sure that the screen is good and the filters o'corse ---> All the fuel line hoses and clamps---> the tank vent line ---> the lift pump ---> fuel pressure relief valve---> diesel purge.
Just search the forum for those key topics for pretty much everything you want to now.
If you are still having low power after that you're gonna have to check cylinder compression---> Fuel injector health---> IP timing and so on..
I think you'll find some answers in the fuel system quest