It could be clogged cats too. Mine were plugged so bad they were causing O2 sensor codes, MAF sensor codes, multiple cylinder misfire codes due to the engine " coughing", and after trying to fix all those and having the same symptoms I found the right cat was plugged and the left was half plugged. Same symptoms though- slam the throttle and it would fall on its face but let up and it went kind of normal. Idled and revved fine when not under load. My mechanic diagnosed the cats for $75 and I had a muffler shop change them for $600. Amazing power now.