Very common for the linkage to have rubber bushings missing or other factors causing the linkage on injection pump to not reach the fully open position. Check it by having a friend prefferably a pretty girl press pedal to floor. Check if arm on injection pump is going to full travel by seeing if you can push it even further by hand at injection pump with engine off of course. If you have a genuine low milage car the timing chain etc are probably still good. Is your car a standard transmission model? Also what year. Wish I had one with about 130 k on it. Needless to say this is just a suplementary check that should be done anyways on these older diesels as what the others posts have stated is the most probable cause. If you have never changed the secondary filter especially. Also do you have a nice clean air filter? Diesels just seem to run better with them although I do not think by any stretch that the air filter is your problem.