3G reception excellent, but dataconnection is not set up automatically...

Since the 2.1 firmware update the 3G reception is excellent. However, I do have a (new?) problem. The iPhone doesn't seem to connect to the data network as well as it used to. For instance, when I step in an elevator the connection with the network will be lost. This is normal... when I step out of the elevator the iPhone will reconnect to the network and I can make and receive calls. However, the dataconnection is not made (automatically). Only turning flight mode on and off recovers or forces the iPhone to make a dataconnection.

This sucks, because without a dataconnection mail is not pushed to my iPhone.

This is not particularly handy. I don't get why this isn't happening all the time and what is causing it. Unfortunately I'm unable to google the problem, because the searchstring is so similar to "normal" 3G problems.

Does anybody have a similar problem or maybe a fix?