Sounds like you have the same problem as me. But mine is actually a triple problem. I'm on the edges of a 2G; 2.5G and 3G coverage triangle so my handset will be running under 2.5 one minute, then it will switch to a different mast a few miles away, which is 2G, but sometimes it will switch to 3G with no signal bars.

It's a right pain in the a**. What I use is "3G/2G/Dual Mode selection Applet" installed from the repositories, and when the network is having a bad day I just switch to the mode that gives the best, most stable connection.

Until the networks invest in their infrastructures, it's all you can do at the mo.