i get this problem quite often. faces are “smuged” with black colour, it might be because the normals are not aligned properly (i recall reading a tut to do with lattices and it said that normals may be misaligned after deforming a mesh and hece black lines could appear). the thing is that i am not using lattices. i am using just a mirror modifier and the rest is basicly extruding, scaling and normal mesh modeling stuff.
The easiest problem to fix in a situation like this is when normals are incorrect. To fix this, simply press ctrl-n and then select recalc normals. However if that does not fix it, then likely there is a worse problem.
It is possible that you have multiple faces connected to multiple vertices in the same location. This could produce artifacts like above if you have AO on. To remedy this, first try remove doubles and then ctrl-n. If that does not work, undo (‘u’ or ‘ctrl-z’) back to before you did the previous two steps. Now go to the offending faces and delete one set of the multiples and it should be fixed (note, you might have to recalc normals after doing this deletion).
i seem to come up with this problem quite often, it might be my computer, but i’ll keep the adivce in mind for next time.
i messed up the model with subsurfs and too much smothing, i am sure there are doubles i havent taken care of coz i split the windshield off. back to the manual to pick up more skills i guess.