Sick Pony

Other causes of a P0300 code:
vacuum leak(air intake, tb, intake manifold, vacuum line, etc.)
bad battery
faulty oxygen sensor(s)
burned exhaust valve
bad catalytic converter(s)
faulty camshaft position sensor
defective computer

On a mechanic specific repair forum I am on here are the confirmed fixes in order of popularity:

• 160 - Coil On Plug(s) (COP)
• 55 - Spark Plug(s)
• 51 - Coil On Plug(s) (COP) and Spark Plug(s)
• 32 - Fuel Injector(s)
• 23 - Ignition Coil(s)
• 13 - Ignition Coil(s) and Spark Plug(s)
• 12 - Engine
• 10 - Catalytic Converter(s)
• 10 - Cylinder Head(s)
• 7 - Intake Manifold
• 7 - Intake Manifold Gasket(s)
• 6 - Mass Air Flow (MAF) Sensor
• 6 - Coil On Plug (COP) Boot(s)
• 5 - Fuel Pump
• 4 - Powertrain Control Module (PCM)
• 4 - Engine Valve Spring(s)
• 3 - Valve Job
• 3 - Spark Plug Wells
• 3 - Rocker Arm(s)
• 3 - Engine Valve(s)
• 3 - Engine Tune Up
• 3 - Exhaust Valve
• 3 - Coil On Plug (COP) Connector
• 3 - Coil On Plug(s) (COP) and Intake Manifold and Spark Plug(s)

I stopped after the rest became repetitive to an extent. What is difficult is it's a random misfire so it makes it hard for you to diagnose which ones are the actual issue right now since it hasn't stored it as a hard code yet for the cylinder it is happening in. Do you have any type of diagnostic equipment or scanner? This can be a tricky diag so you want an experienced mechanic on it, not a random shop.

im curious as well. did you do the spark plugs and COP as a result of misfire issues or did you decide to change to the accel COP and that's when the problem started or vice versa was it the plugs you changed then had issues? the early 05' and 06' cars are very picky on what they like to work with. other plugs are gapped different than motorcraft sp-514 which are .054 or 54 thousands as gap. HT1 and others are .044 or 44 thousands. I would recommend staying with the motorcraft plugs and going back to the oem COPs. take the boots off and use some emery on the contact and make sure the spring is making good contact. put back together and install. also since you've removed them a time or two, I would make sure none of the clips that connect the wires are broke i.e. the tabs. same with the ones on the injectors. I had several of the tabs break and wouldn't keep good contact and had to ziptie them on temporarily till I got new clips. I have an 05' GT and went from E3s to HT1s then back to motorcraft with stock COP and have been running strong ever since. some have reported the champion plugs work good with no issues. report back with info. good luck.