accelerator linkage sticking

I have a strange issue with my '85 300DT. Sometimes the engine responds very crisply to minor pressure on the accelerator, sometimes it takes a heavier foot for the engine to respond. It almost feels as if the linkage was sticking or binding somehow. I checked the accelerator pedal, it was OK, but I replaced it with a new one anyway. No improvement. somebody told me to check a nut for tightness just under the manual shutoff lever, and it was tight. All ball joints in the linkage appear to be greased and move freely. Is there something special that needs to be done to this linkage? Any strategic greasing points besides the balljoints? Like where it goes through the firewall, for example?