Re: What is causing surging idle in Park/Neutral?
Since this all started when you installed a new TB, I believe you'll need to do a "real" electronic throttle relearn. It'll take a good computerized diagnostic tool to do it correctly though. Just pushing the throttle a few times with the key on and engine off won't do a complete relearn. I had similar problems with my BBK throttle body, and I used my Autel DS708 to do the relearn and that solved the problem. Does your cold start idle surge high also? Once warmed up does everything work OK or do you still have the surge?
Your problem with putting the old one back on is that the PCM was already in the process of trying to relearn the new one. Anytime a new TB is installed a complete ETC Relearn should be done. Sometimes you'll luck out though and everything will work fine, but almost all of the time it'll take a while (even after the ETC Relearn) for the PCM to become completely sync'ed to the TB.
05 LX/RT & 06 WK/GCO, modded similarly, MDS "ON" Light, Vortex Gens, Debadged, DashHawk, BT Catch Can, SRT CAI W/Add Air Inlet, K&N Filter, Sonnax Trans Press Booster, SC Tuner W/Adv Trans Tuning App, DS Tuner, Cust tube headers w/3 in. catback, Bilsteins, SRT frt/rear bumpers, Eibach SRT Lwr Sprgs & Sways, FRI Sidewinder Cam Adv 2*, 6.1 valvetrain, SLP 25% UD Pulley, BBK 90mm TB, SOS Coils/.060 plug gaps. 20 in. SRT Reps.