1997 Ford Thunderbird 4.6L P0171 Code: Intermittent Lean Running at Idle

I've already replaced the upstream oxygen sensors with new ones. I cleaned the MAF sensor wires using electrical cleaner, as they appeared clean initially. The spark plugs are brand-new platinum types, and the ignition wires are original MSD units from the factory year. The intake manifold is a PI aluminum crossover with PI intake gaskets and RTV sealant applied at both water passage openings—both of which were previously imperfect. Coolant levels remain stable with no visible loss. I've tested the engine using vacuum port plugs on the upper intake connections to rule out any vacuum leaks in those areas. The driving symptoms are as follows: when the car starts, it runs smoothly for about 5 to 10 minutes. After that, idle quality deteriorates progressively—sometimes with rough running, and at other times it briefly improves before returning a few minutes later. At wide-open or half-throttle, the engine runs strong and pulls well up to 5000 rpm. Between 1000 and 1500 rpm under light throttle, it can run smoothly or exhibit mild lean surges. After several minutes of lean conditions, the Check Engine Light activates with a P0171 code. If the engine runs cleanly for a while, no hard or pending codes appear. However, after running in park at 1500 rpm for a few minutes and experiencing surging, a pending P0171 code sets itself. Even though the right side of the engine registers lean during a P0171 event, all four right-side spark plugs are sooty—indicating rich combustion on that side. In contrast, the left-side plugs remain clean. This inconsistency is puzzling. The injectors sound identical to me and have consistent ohm resistance values. What could be causing this intermittent poor performance at idle and low speeds? Why does the engine run well under heavy throttle but struggle when idling or moving slowly? The issue appears to be specific to low-speed operation, with no significant power loss under load—only drivability problems.