P0306 Fault on 1997 Chevrolet K1500 5.7L Engine - Diagnosis and Solutions

Hello, I own a 1997 Chevrolet K1500 with a 5.7L V8 engine that is currently displaying fault code P0306. I've performed a full maintenance routine including replacing the spark plugs (pre-gapped according to the owner's manual), spark plug wires, oil, oil filter, and air filter. I inspected the pistons using an inspection camera and found minor carbon buildup. To address this, I ran the truck with a fuel additive until empty, then replaced the fuel filter. Afterward, I re-inspected the piston heads—there was some improvement but not significant progress. I've avoided using sea foam cleaner due to concerns about potential damage to the older spider-type fuel injection system. I recently added a fuel system cleaner and refueled. A week later, I changed the oil and filter again. The truck now idles smoothly and runs well—though I still detect a gasoline odor from the exhaust, which is not rotten in smell and no visible leaks are present. My main concern is related to the torque converter: while the engine runs fine, the truck feels heavier than usual compared to other single-cab trucks. I also hear a winding sound when accelerating. When I press the throttle fully at highway speeds (60+ mph), the vehicle does not increase speed but instead just revs up. I'm certain it's not a transmission issue because shifts are smooth, without jerking, kicking, or skipping. I've already replaced the rear differential, which failed during my initial purchase. I'm currently at a loss and would appreciate any insight from experienced users—especially regarding possible causes of P0306 and the unusual driving behavior. The red engine warning light remains permanently illuminated on the dashboard. Any advice or troubleshooting steps would be greatly appreciated.