Throttle staying on

This car is a week old from new, My trouble is?
At first I felt the cruise control was not turning off as the car seemed to carry on at the set speed.
Went to Vauxhall and was taken out on the road and shown how to use it, I was doing the same as shown.
As alway when trying to show what is happening it wont do it.
Went on my way and the same thing happened three four times.
Now I know its not the cruise control as it will do the save in normal driving.
Im driving along 40mph slow down, speed up, slow down, take my foot off the accelerator and the car speed up to 40 mph?
If I break take foot off break it will speed up again.
I was told this model has (wait for it) a free running mode? the car carries on as it feels a strain in the engine and put some power on to keep you going at the same speed.
I dont know that taking your foot off the accelerator should make the car carry on, on its own.
Its behaving like a Herby Car
Any one had the same or having the same problem?
Any fix?
Thanks for any help