Q: Vehicle misfiring and I don't know why

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It feels like my vehicle engine is misfiring, but the Check Engine light is not coming on. The RPMs also jump up and down. I have changed the fuel pump, fuel filter, spark plugs, plug wires PVC valve, and run a fuel additive called HEET through it. I also changed the thermostat for an unrelated problem, and inspected the fuel rail. I do not know what to do next and need professional help.

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