I have been experiencing what seems like slipping in my expedition. It's a 5.4, it started a few months after we got it. At first it only acted up when you would accelerate hard from a stop, it would seem like it dropped outta gear and then it would catch and go like normal. It did this on and off. I could not recreate it when i took it to aamco so that was a bust. One time, and this was the only time, we were headed about 45 mins away and at that point that was the farthest we had drove it. It started to chug violently and a check engine light came on. I was nowhere near a parts store to get it checked out so i went on to where i was going. When I left it was off and it drove fine. tried checking it the next day hoping it would be a pending code but nothing was there. coming to the present, it chugs/slips every time we drive it. so what ever it is it's getting worse. I plan on taking it to a transmission shop close by when i get sometime. Also it's only done this maybe 3 times but there are instances when it goes into some kinda limp mode and seems like it runs on only 4 cylinders. I turn it off and then back on and it runs like new. What makes things so bad is that it wont throw a light so I have know way of knowing where to look, I'm pretty knowledgeable with cars so this has me stumped. I've been hearing that it could possibly be a cop? any truth to that?