It's possible the cat is in bad shape. I'd suggest advancing the timing but it may result in a rise in NOx emissions. Check the EGR system to make sure it's functioning and that the passage in the Y pipe and upper plenum are clear of clogs and buildup. Make sure you're setting the timing correctly, you MUST jumper TE and E1 and set to 10?BTDC. And test the AFM (look for signs of tempering too).
Any mods on the car?
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