I also purchased the visteon coils. Got them off ebay from a seller named "alanisdeals". He sells them for $180. I've been rockin them for over a year without issues. Of course, I used a moderate amount of dielectric grease and even had to apply a bead of hi-temp RTV on the coil covers because the original gasket was not making a proper seal and a couple of the bolts were stripped. We tried to tap them out, and even put lock-tight in there to no avail. Now to the more important issue...
It sounds like you might be experiencing a "surge" at highway speeds. I had this from 55 to 65mph. The best way to describe it is like the tranny doesn't know whether to go from 4th, 5th, or overdrive. If you hold it steady, she will go vroom, vroom, vroom, at 1 or 2 second intervals. Once you kick down on it she will drop a gear and once you get back to cruising speed(70mph) it will be just fine. I originally thought this was fuel or throttle body related.
Let me know if this is what you are seeing. I could be way off, but I'm just trying to help. Lord knows I've gotten tons of help from this forum already

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