Chongo is talking about exactly what happened to my car too. I replaced the entire set of intake manifold gaskets, both post cat O2 sensors and still had those codes. It turned out to be the PVC vacuum line had been sucked flat so many times that a crack developed in it. I replaced that line and everything has been great for about 6 months now.
HTH
96LXV6 heavily un-modded due to family addition.
87Renault GTA convertible, chopped springs, Ford Premium Sound, but otherwise stock.
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