iPhone Reboots after 2 minutes

I have recently fixed a water damaged iphone 3GS by replacing the dock connector, the battery and the lcd screen.

Initially, for about 4 weeks it was working fine with out any issues at all.

Last week it suddenly started resetting when it was charging.

Thinking it would be a software issue, I did a full restore but got the error 29 issue. I looked up possible issues, and ended up getting another new battery. This was worse than before and it would refuse to connect to the pc long enough to restore.

I then ordered another new battery, this time with an early part number, and it was fine for 24 hours, until it needed to be charged, Now it will boot fine, then after 2-5 minutes it will just reset completely.

I have tried airplane mode, and it seems to last a little longer before restarting, but this might be a red herring.

I don't have the original battery so I can't check the part number.

Also, could this be caused by a faulty dock?

Thanks.