Hey guys, I need some expert opinions on what has proven to be a hard issue to resolve. I bought a 2004 Trailblazer 4.2L about 2 months ago. About a week or so after I owned it, it started to seem quite sluggish to me, and it also shifted hard, especially in the lower gears. Then the check engine light went on, which ended up being the thermostat stuck open. I had it replaced, and since then the engine light hasn't come back on, but the truck continues to feel "sluggish", and it also shifts hard, and sort of lunges from one gear to the next. To try and fix the problem, I replaced the air filter, had a new fuel filter put in, changed the spark plugs with AC Delco iridiums, cleaned the throttle body, cleaned the electric terminals and grounds for the battery, bought a new battery, and I also use injector cleaner with every gas fill-up. Everything I just listed seemed to help give the truck slightly more pep, and the hard shifting would lighten, but it is still very much there. I took the truck to a mechanic for an "engine performance test", and he said that everything looked good, and he did nothing but reflash the PCM because I asked him to do that as well. This still did not help, but since he said it was OK, I took the vehicle on a 3 hour trip for a vacation. This is when the transmission went out, and I had a local transmission shop rebuild the hold thing, including the torque converter. After I got the truck back, the shifting lunge is still present, and the truck still seems sluggish to me. For instance, when I'm on the highway, my truck has a hard time accelerating from say 50 to 60 mph. When I say hard time accelerating, I mean I step on the gas and it seems like nothing is happening. I can feel more gas being injected, but the RPMs barely go up, and it barely goes faster. In addition to the sluggishness, it seems like the shift points are sometimes at odd places as well. The only other thing I can think of is that the CAT converter is bad, but I'm skeptic to that idea since my truck passed its emissions test while all of these problems were going on. Does anyone have any idea as to what the problem may be???