Have you ever replaced the spark plug tube seals that are pressed into the valve cover. Oil in these tubes is the #1 cause of misfire according to the FSM. I had a low rpm misfire like you and replacing these did the trick. However, when my HG blew a month later I found two injector sockets broken, bare wires at the EGR and other issues due to age so I replaced the harness.Toyota is telling me the only way to get the plug housing is to buy the entire engine harness. Can you direct me to where I can get just the plug housing?