I have an '09 Aspen with 200k miles that threw a P0307 code 4 days ago. Decreased speed and limped home not knowing what the code was until later. Replaced ignition coil and spark plugs and did an old-fasioned intake leak check with carb cleaner with no change in RPM. Reset code and drove a little over the weekend with no issue. This morning at high throttle the engine misfired but as soon as I let off the gas a little it stopped. Idles normally in driveway. Seeing posts about carbon build up and valve springs but don't know where to go next.