Lumia 925 no mobile internet connection

[mkaminski100]

We have two Lumias 925, one running on stock Three firmware (Cyan) and second is an unlocked Vodafone running Denim. Both have Three card in them.
Both phones tend to loose internet connection randomly. Network coverage is always ok, often showing 4G connection, but there is no internet. This can be fixed by turning Flight Mode on and off, but it?s rather annoying.
This happens in different locations and I never had problems with this when using Android phone.
Could you help me to find cause of this problem?

 

[Aniket Mhatre]

May be bad antenna

 

[mkaminski100]

As per my original question, coverage is always decent with H+ or 4G when this happens, so it’s probably not the antenna. I can also make call at the same time,.
Also if this would be an antenna, flight mode on/off trick would not help.