The Exhaust Recycled Gas (EGR) has gas passages going into the Intake that get block with gunk, causing ''Insufficent flow'' to the EGR.
I picked up a Towncar for nothing years ago with the same problem, the dealership charged the woman $500 for nothing.
The EGR gas passages were blocked, I had it fixed in 20min.
1) Have the local Auto Parts store pull Diagnostic Trouble Codes (DTC) for Free.