Rebuilt Trans in February - 3rd to 4th Gear Problems w hardened installed

Well just heard from the 2nd tranny shop and they said that the TC isn't slipping, but is cycling in and out and needs to be replaced. No mention of TC engaging too soon (which is good and confirms the TPS did what I thought it did).
So I'm going to go pick it up tonight and probably check the spark plugs over the weekend, and then take it to one maybe two more shops for opinions. This sucks.

I went and picked up the car and then went to the original tranny shop and he took the car on a ride with me and let me look at his scanner. I wasn't totally sure what I was even looking at, but I did watch the TCC % and TC RPMs or whatever it was. I didn't notice anything weird based on what he explained to me. The TCC % would start to go up and didn't seem to be affected by the jerking/bucking sensation from the car. He did acknowledge that the TC on too soon issue seemed to be gone though (again by replacing the TPS).
What's confusing frustrating me is that I watched everything on the scanner and it all seemed fine. But the other shop said that the TC would be at 40% and then drop down to 0% and then go back up to 40% with a 200 TC RPM. When I watched it the TC RPM would be at about 100 or less and the TC % would go up pretty smoothly. On the ride-along with my tranny shop we didn't get up to freeway speeds though - he only took us on really steep inclines of like 8%-12% and maybe 45 MPH. I don't know if the other shop went faster than that to see the problem or if that would even make a difference.
Another thing to note, is that while we were on the ride along the check engine light came on. It showed a P0420 (which I would get all the time when I put my DP on) and a P0218 code. I was really hoping that it'd be a misfire code so at least that would give me some kind of direction. The tranny temp code really didn't surprise me because of the extreme grades we were driving on. I never take my car on anything like that, it's just that the mountains start pretty close by the tranny shop and they made for some good conditions to put the car under load. The P0420 has been popping up for two years and I just ignored it because I could clear it and it wouldn't come on again for a long time. I've also seen that happen to a lot of people who mod their exhaust at all. So when I got a tune, that code was set to be ignored. But if a cat or bad O2 sensor might be causing or contributing to my probs, then maybe I should look in to it? I'm not sure.

I checked the plugs and wires, put the old (tuned) PCM back in, cleaned the MAF, visually checked the coils (inspected the terminals/prongs) and everything checked out just fine. Went and test drove the car and no change. Still jerks. I did notice it do it once in second gear though (at least I think it was in 2nd).
With the spark plugs though, I did notice some weird orange coloring on one side of the plug. It has done this ever since I've dropped pulleys but I've had no drive-ability issues and no KR when scanned so I haven't worried about it. I am curious what it is.

Drove the car just around the neighborhood today and when I got back I noticed this really weird sound coming from under the hood (new as of today). It only happens when the car is in gear and it almost sounds like an exhaust rattle of some kind. It's really hard to describe and unlike anything I've really heard before. No clue what it is.

So Tuesday I'll probably take it in to another shop and get yet another opinion.

any updates?

Well last week on the way home from work I lost all gears. It's like the car just slipped into and stayed in Neutral. I called the tranny shop and told him and he had the car towed in to the shop all while still insisting this didn't have anything to do with my "misfire" problem. They tore into the trans and found something exploded in the TC, clogged the filter and starved the trans for fluid. He put in a new TC and had to do a total rebuild all over again. I picked up the car on Saturday and what do you know? It looks like that solved the problem including my "misfire!"

I know this is way to late to be of any help, but have you tried disconecting the C-2 ECM plug @ the ecm and removing pin: 78 (brn wire) from the ECM harness this will, in effect, remove the TCC circut from actuating. I just bought a 2001 GTP 3.8SC and I am experiencing the same problem. The seller did not disclose this and it was not apparent on my too short in town road test. She did show me that the trans and convertor were replaced by GM 66k ago.