C Max Help Required Please

Hi, I have recently purchased a Ford CMax, it’s a 58 plate 1.8L petrol Titanium and has 14,500 on the clock, it’s now out of the trade dealers short warranty.

The issue is at any random time, but seems more on a long run the power will just go, so I am in 5th gear doing 60/70 mph and then the car just starts to decelerate, like it’s going into some sort of safe mode, the engine is still running, but on pressing the accelerator nothing happens it just slows down, i have never let it continue to slow so not sure how slow it wants to run. I then drop the car into 4 gear and the power comes back, then into fifth again and all is fine. It might only do it once on a 30 mile run, or sometimes not at all.

No engine warning lights come on, so not sure if it’s registering anything that a diagnostic run would pick up. It starts fine, no loss of power pulling away.

Any ideas were to start, I have read diesels losing power, but can’t find anything on petrols.