Was that a P0300 code or was it another code?
How many miles on the vehicle?
Does it miss at idle or does it just idle up and down?
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Your next tasks are to check fuel pressure with the key on---engine running at idle----engine running at 2500 rpm. If its low at any point then the pump and or filter or regulator are faulty.
Next you need to use a sophisticated scan tool and monitor power balance and see if its one cylinder or all of them misfiring.
You then need to run a compression test on each cylinder to see if any are low from valve train fatigue or headgaskets or engine wear.
Fuel pressure changes up and down at different rpms. Yes you can have low pressure issues at idle. This may not be the problem but it is one check which must be done.
Coils usually cause a misfire around 45 on hills while lugging. This is when weakness starts to show up first. If you feel its the coil causing this then replacing it is a cheap option to try rather than performing other diagnosis steps.