I had a buick that would do that, and it turned out to be a bad computer chip. THe main chip also controlled the cruise control, and when the chip went south, so did the cruise. Also, I've had this happen on another car when a vaccum line was leaking. The cruise control uses vaccum to set the speed and accelerate, etc. I don't know if the x would have these type of systems, but a vaccum check is pretty easy, anyway, and a computer check can be free at some auto parts stores. Hope you find the problem, I don't want to drive behind you if you keep speeding up and slowing down!

 

I had the same problem.... turned out that it was my brake controller that was bad and it kept kicking the cruise out. Only took me a year 5 mechanics and myself to figure it out.

 

There is a recent Ford recall on the speed controller(cruise control), you may want to check it out.

 

I went to dealership and checked for recall and was told that there was no recall on my truck. 2 weeks later I got a recall notice. Any way my fix was adjusting the switch at the brake pedal it turns out when I hit a bump the pedal would move enough to shut off cruise.

 

Gotta love the dealerships. NO recall. or where they talking about the year of your x. I it would be understandable if the recall came out a week ago. But it has been out a couple of months. Took mine in and it took ten min to fix. I think all the did is add a fuse