Ok, found out the problem. I had a bad fuel system (which I told my mechanic that was the problem) but he didn't diagnose it until I came back a third time with a rattling catalytic converter. I got a new fuel system for $370 installed and he told me to take it to the Ford house to have the 80k mile federal emissions warranty work done to replace the cat for free. But since I have an aftermarket cat, it is going to cost me $180 at my trusty muffler shop for parts and labor (it is the same cat from the SHO shop). The cat's ceramic coating on the inside was killed when the bad fuel system started burning too much fuel inside the cat. So all in all it looks to be $550 for a f'd up'd fuel system.
