2005 Ford Focus 1.6 TI 115ps Petrol Engine P0335 Fault: RPM Gauge Drops to Zero at Idle, Misfire Occurs

Hi everyone, I'm experiencing a puzzling issue with my 2005 Ford Focus 1.6 TI 115ps petrol engine that displays fault code P0335 — Crankshaft Position Sensor Circuit A Malfunction. The problem only appears when the car is idling. When I start the vehicle and drive for about 5 miles, everything operates normally: the RPM gauge reads correctly, and the MIL (Malfunction Indicator Lamp) stays off. However, after stopping and idling for approximately 5 minutes, the RPM gauge suddenly drops to zero, the MIL light turns on, but a scan tool shows around 75 or 750 RPM — indicating the engine is actually running. The engine either runs smoothly or begins to misfire slightly. I've already replaced the crankshaft position sensor and swapped both camshaft sensors (inlet and exhaust), as this engine model has two camshafts with dedicated sensors. Despite these replacements, the P0335 code persists. My key questions are: 1. Why does this issue only occur during idle? 2. Which sensor does the Powertrain Control Module (PCM) primarily use to determine true engine RPM — the crankshaft sensor or one of the camshaft sensors? 3. Disconnecting the crankshaft sensor causes the engine to fail to start or stall immediately, suggesting the circuit is functional. However, disconnecting the inlet camshaft sensor results in a slight rough idle, while the exhaust cam sensor has no noticeable effect. I'm looking for expert insight into whether this could be a wiring issue, signal interference, or a fault in how the PCM interprets signals from the sensors during idle conditions. Thanks for any help — Steve.