Car struggling to turn,rumbles,and trouble accelerating?

I have a 98 Ford Mustang with a flashing check engine light, took it to Autozone and was told there's a misfire on 5 (spark plug?) I noticed the car was then having trouble turning on after cranking. Along with that the engine rumbled making the whole car shake and had trouble accelerating. I then installed new spark plugs and spark plug wires. I also installed a new alternator because the old one was going bad. After that the car was running fine. Till just recently its happening again, its struggling turning on, when it does come on the car rumbles a lot and doesnt have much power when accelerating. Any info on what it could be and how to fix it please?

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