Ive seen the EGR pipes that wrap around under neath the plenum become so clogged with carbon, the carbon has to be drilled out. The pipes are some what accessible but I haven't had to do so myself yet. My EGR code would come and go. Turned out the vacuum lines on the passenger side of the plenum, one goes to the intake hard pipe, one goes to under neath the throttle body, were reversed.
Alternatively if your motor can take it, Italian tune up, blast that carbon outta there.