Had a look at the log and found lots of these errors:
08.01.14 06:53:59 Timeout during PPP negotiation.
08.01.14 06:53:59 Internet connection cleared.
And as I know, web surfing / Youtube / speedtest etc are ok as long as the speed doesn't exceed 1.5m for a certain period of time (5 minutes maybe).
If I start to download something big, it will disconnect / timeout.
Hope Snap/Chorus will bring a fix soon.