My parts came in yesterday and went ahead and installed the fuel pulse damper this morning. The short term fuel trims went more negative as expected from the previous install. The car runs more smooth but the stumble is still there, but this time instead of stumbling hard at 3000-3500 rpm and triggering a P0303 with the CEL staying on, it shifted the stumbles back down to 2600-3000 rpm triggering the CEL, but it goes on for less than half a second and then stays off. Below 4000 rpm, the short term fuel trims go negative as low as -15%. I wouldn't have thought that a damper can have such an effect on afr that much.