My prior post was my resolution. A hole in the fuel map frequently encountered while on track caused the FT to go way off. This either directly affected idle or fouled the plugs and affected idle. In either case, I chalked it up to a tuning issue.

Oops, I totally missed your "conclusion."

I've yet to fix this issue, as it only seems to happen on the track in warmer/hotter ambient temperatures (I'd say over 65 °F) with few 20- to 25-minute sessions. Yesterday, however, it happened immediately near the end of my first 20-minute session in the morning. What's interesting is that this issue disappears after the car cools down (not driven for a few hours).

I spent the rest of the day in the paddock trying to troubleshoot the problem -- resetting the ECU with idle relearn procedure, plugging in a code reader to look for any abnormalities, reading live data with the same reader -- all with no success. I didn't run any other sessions because the idle kept "hopping" and the engine decided to stall intermittently in the paddock while idling. Interestingly enough, this issue disappeared after I packed everything up and started the car (about 4-5 hours after the problem occurred).

Here's what I have so far with my specific problem:

• Car is not tuned
• MAP and IAT values are in the normal range
• Fuel trim reads out of range only during the idle issue
• New OEM spark plugs were installed (OEM) in the middle of the season, but this problem occurred even with the previous OEM spark plugs

Other details:

• Koyo radiator installed before the start of the season -- realized the issues started occurring since the start of this season
• Valve adjustment was performed during the middle of the season -- issue was present before and after this

Would love to see if other people found solutions to this problem. I know of at least 3 other friends that I track with, who are experiencing the same issues here in the Northeast.