2003 Hyundai Santa Fe Rough Idle and Misfire: P0300, P0302, P0304, P0306 Codes - No Power, Stalls After Rain

I have a 2003 Hyundai Santa Fe with a 3.7L V6 engine and all-wheel drive. While driving today, the vehicle suddenly became gutless—idle is rough, and it lacks power. Using an OBD-II scanner, I read codes P0300 (Multiple Cylinder Misfire), P0302 (Cylinder 2 Misfire), P0304 (Cylinder 4 Misfire), and P0306 (Cylinder 6 Misfire). These indicate misfires in multiple cylinders, specifically affecting the front half of the engine (cylinders 2, 4, 6). I replaced the spark plugs with NGK iridiums last month. I also installed a new PCV valve and a rebuilt alternator, and recently upgraded to new battery terminals. The car ran smoothly for about a month but then started losing power. Now, when the engine is off, it reads 12V; upon starting, voltage spikes to 14V, then drops to 10V after shutdown—requiring several minutes before it can restart. The vehicle has been maintained with regular oil changes, new air and oil filters every three months. It runs fine when warm but becomes rough during heavy rain or cold conditions. After being left idle in the rain, it misfires until driven for a while to reach normal operating temperature. Previously, this issue would resolve itself after warming up, but this time, even after driving, the problem persists. I’ve confirmed spark is present at each plug, and I checked the PCV valve—no signs of blockage. The codes return immediately after clearing them. I’m concerned about whether the engine is receiving sufficient spark or if there’s a deeper issue like fuel delivery, ignition timing, or sensor malfunction. Any advice on diagnosing this misfire pattern in a 2003 Hyundai Santa Fe with a 3.7L V6 engine would be greatly appreciated.