I have a 97 mazda mx-6 it sounds and feels like a lifter is hittng one plug

[James]

the car lost half its horsepower.? I let my friend drive my car and when i got it back it was running really rough so I thought maybe fuel filter, not that. Then i thought maybe the converter but I was under the hood and only one of my plugs feels like something is hitting it. the car has lost about half its horsepower and is kinda shaky now. I have no clue can someone Help Please