2004 Toyota Highlander V6 - P0351 to P0356 Ignition Coil Faults Causing Engine Surging at Idle

I've been maintaining my in-laws' 2004 Toyota Highlander V6 3.0L 2WD, and recently noticed it began surging while driving. After towing it back to their home, I started investigating the issue using an OBD2 error code reader. The vehicle displays multiple check engine lights — Check Engine, ABS, and VCS — and shows the following diagnostic trouble codes: P0351, P0352, P0353, P0354, P0355, and P0356. No other pending or active codes are present. Symptoms: - The car runs smoothly when cold-started. - As the engine warms up, idle drops, and the moment I press the accelerator, it begins to surge between 1000 and 1500 RPM. - When releasing the gas, the RPMs return to idle. However, after letting the car cool down and restarting, surging returns within about one block. Troubleshooting Steps: - Prior to getting a code reader, I suspected a faulty throttle position sensor or a clogged throttle body. I removed and cleaned the throttle body (which lacks an IACV due to being electronically controlled). Coolant flow through the tubes was normal, indicating no internal blockage. - After cleaning, the issue persists. Current Diagnosis: Given that all six ignition coil fault codes (P0351–P0356) are present and the symptoms occur only after engine warm-up, I believe this points to a failing ignition coil set. Since the car starts and runs fine when cold but surges under load once warmed up, it strongly suggests that one or more of the six spark plugs are not firing properly. Next Steps: I am now considering replacing all six ignition coils (one for each cylinder) as a full system fix. If this doesn't resolve the issue, I will perform additional diagnostics such as live data readings from engine sensors (e.g., crankshaft position, throttle position, and knock sensor), which I can share if requested. Any insights or alternative diagnoses for P0351–P0356 on a 2004 Toyota Highlander V6? I appreciate any help or suggestions.