Have to say I agree with others, sounds like headgasket, had to have mine done 5 or 6 years ago on my '05. Started with a rough idle at start and would eventually relax, only seemed to be when engine was cold. Eventually it got worse, check engine lights came on and started to see white smoke out exhaust. If you have a check engine light on run the codes, if you have a misfire on 4 or 6 (like stated above) you most likely have your answer, headgasket. You could try swapping the ignition coils to see if the misfire changes location but if there is loss of coolant I doubt that a faulty ignition coil is the issue. As for the sealer not working I'm not surprised, everything I read about those seem to be hit and miss, depends where the leak is and really isn't a long term fix. If it is the headgasket get it fixed properly and you won't regret. I'm happy I spent the money and committed, has run great since and very happy to still have it around.