Misfire in cylinder 4 and 6

I have a 2003 Kia Sedona, the check engine light has been on for some time now. When read it says a misfire in cylinder 4 and 6. The problem I am having with the car: It misfires as you back up and when the car is in idel. Also it has a very rough go from the idel position. These last week it has actually started shaking as the van is being driven. We have cleaned the fuel injectors, moved coils around and reaplaced spark plugs. We have check the timing belt and while it is slightly stretched the timing is correct and lining up each time it is tested. We have also ran a compression test which is comeing back normal. What is the problem with the car?