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This is very likely a restricted catalytic converter. What happens is the restriction in the exhaust causes incorrect airflow through the engine. The air can't come in because it can't get out of the exhaust. Therefore the readings the mass airflow "should" see related to engine vacuum, rpm and load are slewed off from where they should be. This often shows up as a mass airflow sensor fault code even though the restriction is in the exhaust. A test with an exhaust backpressure gauge will confirm this. I've seen some techs temporarily remove the oxygen sensor and drive it (albeit very noisy) to see if there is a difference in power. Based on your description I'm as confident as I can be from my chair that a restriction in the cat is your problem. I would suggest however confirming it either way suggested above as they can be a bit expensive and we want to be sure. :)
It often happens that way. There may be a partial restriction for a while but eventually it just gets to the point where it affects engine operation. Generally you're looking at an hour and a half or so plus any diagnostic charges or broken bolt charges. Again... I strongly suspect that's your problem but please do confirm it with the backpressure test or the other way. Replacement of the cat at home is not always the easiest thing to do for two reasons. One is the angle it needs to be moved to in addition to moving the trans crossmember is all but impossible on the ground. Secondly is the nuts/studs that hold the converter onto the manifold quite often are frozen on and may require some creative measures to get them off... sometimes air chisels... torches, etc.
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