My cruise control seems to have developed a problem. At higher speeds , say 70 something when I set the cruise control and I have to drop to a lower speed, after the car has accelerated to the set speed it reaches it and then it is as if I have deactivated the cruise control setting and the car just slows down. I can test this repeatedly. I haven't experienced this at lower speeds.

I am wondering if this is a "feature" i.e. trying to stop you setting high speed cruise settings or a fault ?

Any ideas ?