81 300D

I've seen alot of discussion on engine running rough and pinging/clattering when RPM's raised. The advise was it most likely is an injector problem.

I have just the opposite problem. My engine runs smooth as silk above 1000 rpm, no misses that I can tell.

However, when rpm drops to idle then I hear the pinging and nailing, it also starts idling rough, jumping and missing.

Same problem? Anything particular I need to look into? Or Should I just expect this with 240k mile engine?

Note: I did disconnect each injector, one at at time while idling rough. I could tell the engine would run even worse on each one, so each must be all working, the pump was pumping, however, when I disconnect 1 injector I didn't hear the Ping as bad.
I also ran LM diesel purge thru it, no change.

Thanks