Hello to everybody and sorry for my poor english, y had the same problem whit my iphone 4, it kkeeps rebooting every 2 minutes, but i fixed, y read a lot here and in other sites and try every thing, change the battery, restoration in dfu mode etc.
The solution of the problem was the battery connector, it has 6 solder points , 4 in te rear and 2 in front,the 2 solder of the front was unatached, so to fixit i had to re solder all the conector to the board, and thats it, problem solve, and my phone never reboot or crash again