Had that 2 weeks ago on my '03 Ruby (17k mi.). On way home from work it started missing real bad and light came on. Dealer was on the way home so I dropped it off. Talked to the service mgr. & head tech. He said a lot of check engine code calls are coming in this time of year, with the change over into winter gasd blends. He said mine was throwing misfire codes on 5 of 6 cylinders. Tech said 4 liter valves do not auto-spin / rotate until you get to about 4000 rpm (they should do it at normal driving RPM's). Valves should rotate a little with each stroke to help eliminate carbon deposits. Mine was all gunked up with carbon, and that then got all over the plugs. They took the vlv cover off, manually rotated / cleaned up the valves, replaced spark plugs, cleaned fuel injectors & intake, throttle body, and put an insulating jacked on the injector for #3 cylinder (said something about gas pooling in there). I had to pay for the plugs, but the rest was warranty work. (TSB's on the insulator and valve clean-up). Tech said to run as low an octane as I can without pinging, (which mine does a lot with 87 & some with 89). Higher octane means more carbon build-up.
He actually said I should drop it in low every once in a while, 1st gear, & drive it up around 4000-4200 rpm (to get the valves to spin & self-clean). He said you can't baby these engines, they're rock solid & like to be ridden hard (need it). This was the Tech!
I did it once, she was screaming!
Been running great since they fixed it.
Hope info helps