iPhone reboot, error 29, gas gauge errors

That error could indicate a hardware issue. Have you had the iPhone in a place where i can become overheated? Have you tried restoring completely wiping all apps/info off the phone? What type of voltage battery are you using (3.7V-3G/ 3.7V 4.51Whr-3GS)?

Take a look at this website from Apple, it describes in detail on what each error means and how it can be resolved.

http://support.apple.com/kb/TS1275

Usually when you receive possible hardware errors it means something, somewhere has stopped working. And my guess is that it is the logic board. As with all electronic devices ever made, they all have the slight chance to stop working for absolutely no reason at all. I bought a brand new video camera once from JB HI-FI, got home and for some reason it would turn on and off when it felt like it from the very first time i pressed the power button. Why? Who knows. But logic board hardware issues are pretty much impossible to fix unless you have very skilled soldering skills and aloooooot of patience.

If you have gone through all of the steps on the apple link, then it doesn't look good. I wouldn't bother buying a new logic board either because it will most likely be someone else's faulty part they wish to sell off cheap. Personally i would go and buy a cheap Nokia - nice and cheap, very sturdy and reliable and wait until the launch of the beautiful iPhone 5 :)