I have a 99 gtp and have gone through the EXACT same thing you have, first thing, did you do the crankshaft position variable relearn after replacing the sensor? If not start there, if so, you'll be right where I am, I had cat and "u-bend" cut out, I've replaced plugs and wires, and I'm still having a massive misfire/stalling/sputtering issue, taking it to my local gm dealer, it'll be pricy but I'm sure they'll find the issue, anyways, I will keep you posted on what I fin out if you would be so kind as to doing the same, I've been chasing this problem for two months and throwing money at the wrong stuff, as I'm sure you have too and would rather not anymore, chow for now