2006 Ford F350 King Ranch 6.0L Diesel - P0404 Code: White Smoke, Sputtering Under Load

I purchased my 2006 Ford F350 King Ranch with a 6.0L diesel engine about a year ago from the original owner, who was very diligent about maintenance and repairs. For the most part, the truck has performed well until approximately one month ago when I began noticing engine sputtering and surging under load—this occurs only during normal driving conditions, not while towing. Additionally, white smoke is now being emitted from the exhaust pipe. The issue temporarily resolved for about two weeks before returning with full intensity. I recently used an OBD-II code reader to check the system and found a persistent P0404 diagnostic trouble code (DTC), which indicates a malfunction in the diesel particulate filter (DPF) or exhaust gas recirculation (EGR) system. Below are the relevant sensor readings: - Engine Speed: 688 RPM - Barometric Pressure: 29.20 psi - Calculated Load: 32.6% - Mass Air Flow (MAF): 1.93 g/s - Manifold Absolute Pressure (MAP): 29.5 kPa - Coolant Temperature: 174°F - Intake Air Temperature (IAT): 41°F - Acceleration Position D: 19.2° - Acceleration Position E: 29.0° - Acceleration Position F: 18.4° - EGR Error Rate: 71.9% - EGR Command: 29.8% - Continuously Variable Voltage (Cont Mod): 13.8 V - PTO Status: DiSE Given the recurring symptoms and the presence of P0404, I suspect a buildup of carbon deposits in the EGR system or a partial blockage affecting exhaust flow. The white smoke may be linked to incomplete combustion due to poor EGR performance. Any insights or troubleshooting steps for this specific issue on a 2006 Ford F350 King Ranch with a 6.0L diesel engine would be greatly appreciated.