This sounds very similar to what I might be having. Earlier this year I had an intermittent CEL until around Sept the car started to bog down while cruising and stall out at lights. It's already seemed more sluggish than two other Envoys I've driven in the past two years and thought maybe I had different gearing in the back and my "diesel" sound is also louder than those two. I had one mechanic run a test and it was the cam sensor, so I replaced it, but it still did the same thing. Went to the parts store, they tested it and that sensor was good, but one in the tranny (don't recall what) was now the problem. Took it to a shop to have that swapped but while they ran everything to make sure this was it, they got other codes when the engine acted up. Shifting was sluggish or didn't downshift when applying power until you floored it, error codes seemingly would switch to different problems or none at all, they said. Also said the cat was blocked. I decided not to do any repairs, as the mechanic's personal opinion was some sort of electrical issue and it was time to sell it, repairs and costs would start being more than it was worth. First mechanic had a new theory that the PCM was going bad. Figured if codes would shift around, it wasn't working correctly being the "brains." So I got another used one, installed it, reset things, but same issues. Finally the truck has been in my driveway for the past two months as I've debated what to do with it. Since then I've found another mechanic who really seems to know his stuff after correctly diagnosing and fixing a bunch of work cars and suspected my issue could be a combination of injectors and the fuel pump (which the one shop also said was weak). He said the 02s had injector problems. He spent the last week testing, driving, etc. but of course it wouldn't stall or have the issues I've been having. But his diagnostics finally showed bad upper O2 sensors using the replacement PCM I bought and a bad TPS on my original PCM (which I know has the latest firmware after having it flashed two years ago). From what I'm reading here, sounds like the O2s could be the problem (or maybe one of the problems). I've been debating to get rid of the truck with 185K now on it (I got it with 125K) since I know there are other issues, but if the O2s solve most of this, I may be keeping it longer. Any thoughts why two different PCMs would show two different problems?