My iPhone 1st Gen was having this problem before when it's still on firmware 1.1.4. What I did was DFU restore to firmware 1.1.4. Since my iPhone's bootloader was 4.6, so I downgraded it to 3.9FB using iLiberty+. After iLiberty+ downgraded my iPhone bootloader to 3.9FB, it automatically reflashed it's baseband to 04.04.05_G. Click here for more info.