Nissan Maxima 1997 P0304 to P0306 Misfire: Coil Replacement & Root Cause Diagnosis

I'm posting on behalf of my roommate, who is currently facing a persistent engine misfire issue with his 1997 Nissan Maxima. The vehicle features a 6-cylinder automatic Dual Overhead Cam (DOHC) engine and has surpassed 200,000 miles—still running but requiring attention. The car previously had cracked valve cover gaskets on both the front (near headlights) and rear (far from headlights) covers. This led to oil seepage and smoking. We recently replaced the front gasket manually with care; it's now functioning well. The rear gasket will be addressed soon, as it is more difficult to access. The primary concern is a recurring misfire: initially cylinder 4 (P0304), which we diagnosed by replacing the coil pack—this resolved the issue temporarily. However, just 3–4 days later, a new misfire has appeared in cylinder 6 (P0306). The engine now runs roughly when idling in drive or reverse, with symptoms similar to the original misfire. We're wondering if there's an underlying cause beyond individual coil failures. Could this be related to the valve cover gasket leaks? Is it possible that a deeper mechanical issue—such as piston wear, fuel delivery problems, or internal engine damage—is causing multiple cylinders to misfire? Given our limited budget, we’re seeking clear, actionable advice and a prioritized checklist before proceeding with further repairs. Any insights into common issues in 1997 Nissan Maxima engines would be greatly appreciated. Thank you for your help—this is a very frustrating issue to troubleshoot without professional tools or diagnostics.