Seems to be similar to this: 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.
The only way to get the phone back online is to power off the device (toggling flight mode sometimes helps).

The problem seems to be related to when phone does a lot of switching between GSM <-> 3G as locking the phone to only GSM mode (or only 3G-mode) makes it work better.
I have also noticed similar behaviour when travelling and the phone frequently has to switch between different cell towers.

Nowadays I use ProfileMatic to, automatically, lock the phone to GSM-mode when I am in certain areas were I normally have this problem.
Since I started with this I have had almost no connection issues. But it is a little sad to, in the year 2012, have a phone without 3G connection.

//j