An unknown error occurred (28) message appearing when you try to restore using iTunes seems to be a common issue affecting iPhone 3G and 3GS units.
This is primarily caused by a defective NAND (Not AND) chip or the internal flash memory. However, there are several software fixes that may work. You can try one of the followings and do a full restore. You can also try this alternative solution which requires you to disassemble your iPhone.
Wait for a couple of hours before doing another restore.
Drain the battery completely.
Bring the iPhone to DFU mode (not recovery).
Use another USB port on your computer.
If you are willing to assume the risks, you can try dismantling your iPhone, disconnect the logic board from the battery, and then reassemble it back together.
If none of the above works for you, then you may want to take your iPhone to the nearest Apple Store. If yours is still under warranty, they can get you a replacement. Otherwise, you may consider using our service as we can fix this issue by replacing it with a new NAND chip.
Please bear in mind that all suggested resolutions above are based on the assumption that you are not a jailbreaker/unlockers. If it works – you may risk losing your jailbreak and/or unlock.