Could be,A fouled/worn spark plug(s),
A faulty spark plug wire(s),
A bad distributor cap and rotor,
Incorrect firing order,
Moisture inside distributor cap,
A bad fuel injector(s) if applicable,
A burned valve(s),
Contaminated fuel (water in gas),
Incorrect fuel pressure,
Dirty/plugged fuel filter,
A failing fuel pump,
A vacuum leak,
The year, make and engine info would help for more possibilities.
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