Many users have been reporting problems with the iPhone firmware 2.2 upgrade from Apple. It may be that the best way to solve these problems is to take a step back and downgrade to firmware 2.1. Others inadvertently upgraded their phones when syncing to iTunes on their computer and for whatever reason would like to revert.
The process is relatively simple and should bring back the 2.1 version OS to your iPhone. Performing these steps on an iPhone 3G may work, however this article does not cover reversing the 3G baseband update included in firmware 2.2. Remember, taking these steps is at your own risk, and any hacking of the iPhone could render your remaining warranty invalid.