I went through the same ordeal with my 4Runner a while back. I took it to the dealership and they hooked up the computer and told me that it was the Bosch plugs and replacing them with factory Toyota plugs would fix my problem. I followed their suggestion and left the dealership happy with my 4Runner running great, but my back pocket $125.00 lighter. Well, one week later, my 4Runner went back to doing the exact same thing as before the dealership replaced the plugs. I figured there was only one other possibility as to what was causing my problem, so I ran out and picked up a set of plug wires and slapped them on. The 4Runner fired right up and has ran great ever since. This is just my experience, but I hope it helps.