Definitely sounds like CAT issues. Literally just went through this myself. I had no CEL and scanned three times on three different scanners. No codes whatsoever. Since late January it was feeling as if I was constantly driving into a headwind, especially from a cold stop. The car blew codes in Feb for misfire and I replaced all the coil packs. They were in terrible shape. Cleared codes, and everything was fine until about two weeks ago. Car started and idled fine, but under even the mildest acceleration, it would act very skittish, like she couldn't get her legs under her. Going up hill was a task, and even when she would get up to a respective speed, trying to give her some juice to pass a car on the highway would drop the RPM's and felt like I was towing something behind me. So with no CEL or codes to go on, I figured since I replaced the coil packs, that it was time to go balls out, and replace wires, plugs, and the throttle position sensor. These made absolutely no impact. Tranny fluid was a bit low, but was still red, and it didn't smell burnt up. I cleaned the MAF, and did a thorough inspection under the hood for any cracked vaccuum lines, and brittle connections. All good. My fuel pump and fuel filter are just about a year old..so I didn't suspect anything there. So, I dropped her off at my mechanic, and said "CHECK THE CAT!"....low and behold, and $650 later, I have a new Walker CAT with warranty, and the car hasn't performed this well I suspect, since it was new. We had a pretty brutal winter here in NJ, and the roads have been torn up pretty good. I recall hitting a nasty pot-hole that literally felt like the fillings in my teeth were going to fall out. I suspect that hit had a bit to do with putting the finishing touch on my CAT. Could have knocked some substrate loose on an already aging part (the original, with 123K on it) Car is a 2001 GP GT, 3.8L. Bad CAT's don't always throw codes either. I had a similar issue with my 02 Bonneville. Definitely check the CAT.
