Its a known bug which Nokia never fixed. Listed here (cached version as the original site has been taken offline):
https://harmattan-bugs.nokia.com/show_bug.cgi?id=79

Phone drops network connection without any visible indications shown on screen, the signal strength meter can be full and 3G icon active but the phone has no connection with the outside world.
The only way to get the phone back online is to power off the device (toggling flight mode sometimes helps).

From what I learn this problem often occurred in areas where the phone had access to a lot of different cell-towers but where these cells also was under quite heavy load due to many simultaneous users.
I also noticed similar behaviour when travelling and the phone frequently had to switch between different cell-towers.

The "Nokia Link" update is said to make it better but I still had this issue after that one installed (but maybe a little less frequent) .

The solution for me was to use the phone in GSM mode when in these problem areas.

To not have to use the phone in GSM mode all the time I used the excellent program ProfileMatic. By creating a rule which detected ID-numbers of the cell-towers in the areas where the phone used to drop connection I could have ProfileMatic automatically toggle between GSM and dual mode.

//j