2002 Ford Mondeo car stutters sometimes while driving.

Car stutters sometimes while driving. Not all time but seems to be getting slightly worse and more often. Tried fuel additive and got a cloud of black smoke out exhaust when revved up but still stuttering. Fitted new oil, air and fuel filters. New Oil. Cleaned Injectors. No warning lights on and no error codes on test machine. Seems to go away when I accelerate slightly but comes back a few seconds after car settles back to normal speed. Very annoying. Any Ideas what can be causing it?