I have P1491(Insufficient lift of EGR valve) on 96 Accord LX ( 2.2 lts , 4 cylinder), I did cleaned the intake ports and the code went away for 3 months, came back again, I have inspected the port , they are clean, measured vacuum as well, and found no apparant leak in tubing. changed the EGR with new one(think it must be sensor), but still has the code , what else should be tried?