Acceleration Problem

I have been facing a weird issue with my Fiat Linea 1.3 Multijet Emotion Pk. Occasionally, when on a long drive on highways, I have noticed that the car looses all acceleration for a short period.

At this point, there is absolutely no response from accelerator or gear shift (downshift or otherwise). It is as if the vehicle gets stuck at that particular RPM and speed with only slight deceleration.Obviously, on braking the vehicle, it loses speed and there is no issue with braking. Also, Upon aggressive downshift (downshifting to achieve RPM above 3500-4000), I am able to regain normal acceleration.

There is no error in diagnostic software when checked at service centre. Further, it is very difficult to replicate the error in workshop due to unpredictability of this problem.

One more observation Car doesn’t give sudden response to acceleration while running in high speed (above 120kmph). It is happening in long drive.
After stopping car for few min it behaves normally for few km but then after few km same problem and after few KM normal behavior.
It is very inconsistence problem. Unidentified by sensor scanner at service center. After servicing of vehicle also same problem persist.

Please help me to resolve the problem