So after the last datalogging session, I did another test drive.
Everything mostly normal, except..... when cruising at 80-100 km/h (50-60 mph), the engine started acting up like crazy. Stalling, choking, tachometer jumping up and down, etc. Not just the tachometer and engine, but the car itself would also slow down, accelerate and "jerk back and forth" at the same pace with the engine. So apparently the TCC was locked when this was happening.
I pulled out from the "highway", stopped the engine and tried to start again. It started roughly; choking, stalling, etc. After detaching the EST bypass cable, I was able to drive it home carefully. The engine seemed to run better with the EST bypass disconnected, but I can't tell if it was normal for that condition. The exhaust sound was more "bubbly", and I didn't see the tachometer jumping around anymore.