There could be many reason for Check Engine Light to flash..................Since you have P0300 code for Multiple Cylinder Misfire Detected, that means you have more than one cylinder misfiring ..............Are you sure there was NO any other fault code pointing to any cylinder fault.............?

You need to check again.................

1) Cylinder Compression Test
2) Check Your Coil Packs
3) Check your Fuel Injectors
4) Check for any Mechanical Faults

Hope this helps.......................

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 What is the mileage done this vehicle.............?  Have you check if S/Plugs are not oiling up...............Which other fault code you got.................?

no idea

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 Lets backup a little. What engine are you working on first. You say you have moved the plugs but what about the ignition coils? When does this misfire happen at idle only or at any speed. If it is a idle only misfire then smoke the intake for port or injector seal leaks on the effected cylinder. What sort of fuel trims are you running? Also if it is a misfire only at idle a lean injector can also cause this. Most misfires caused by local vacuum leaks and slightly lean injectors will go away at off idle condition.