P0401 Code Repeated on 1996 Neon Highline - EGR System Diagnosis and Fixes

In the spring, my 1996 Neon Highline began experiencing catalytic converter pipe collapse. I replaced it with a Walker 15470 (part number 15473), and all exhaust components were subsequently replaced. Shortly after, intermittent EGR-related error codes started appearing. The original converter had a front-end hole, which I initially attributed to exhaust leakage disrupting the engine control module's long-term sensor expectations. About six weeks later, the vehicle began bucking on slight downhill grades. I considered this either a torque converter lockup or an EGR issue—until it started consistently throwing EGR-related error codes. I replaced the EGR valve and transducer with an Advance Auto unit. Despite this, 'insufficient flow' errors persisted. I then switched from the original foil gasket to a more robust Felpro gasket, which improved performance slightly. However, the car now alternates between displaying P0401 when driven on highways and shutting off under light load conditions. When the MIL (Malfunction Indicator Lamp) turns off, the vehicle hesitates and bucks noticeably on slight downhill grades when accelerating—this behavior resolves when I release the gas or drive uphill and then re-accelerate. The symptoms seem counterintuitive: hesitation disappears when the check engine light is off, but returns with a strong bucking sensation when the light is on. Last week, I disassembled the EGR system and inspected the valve, tube, and ports. The tube showed only a light carbon buildup, and I was able to run a clothesline wire through it without resistance (aside from curves). The valve opens properly under vacuum. I tested airflow by blowing directly into the port—air passes freely. A fiber optic camera failed to penetrate more than half an inch due to insufficient lighting; no visible obstructions were seen. A flexible wire was inserted about 2–3 inches with no resistance. The hoses are routed as originally specified: bottom hose connects to the bottom of the transducer, top hose to the top. I’ve researched EGR system operation and learned that the engine control module expects specific exhaust pressure thresholds when the EGR opens. To verify this, I have a downstream O2 sensor installed (the upstream one was replaced in January 2016). At this point, I’m at a loss—what should I check next? I suspect either a defective Advance Auto valve or an issue with the exhaust system routing. A key detail: I did not replace the iron ring gasket at the top of the catalytic converter where it connects to the exhaust manifold, and it was slightly loose. I have an OBD2-to-USB cable and am open to any diagnostic suggestions—what’s next in troubleshooting this P0401 issue on a 1996 Neon Highline?