I just had this same thing happen to my 03 SSEi two days ago, checked the plugs and wires and all checked out. Dan suggested I check the coil packs and turns out my 2 week old coil packs had gone bad. If you have checked everything else give that a try, they are very easy to replace.
I might take the valve cover gaskets off and check all the fuel injectors and possibly replace the o-rings. I know my state was very vague but again I haven't had time to actually take a look in the engine bay. My engine has 137k is it a good time to replace the injectors?