I have just had same issue heres the steps I did.
1:Make sure your battery is fully charged or at least 50% or it will not go into update mode. Even if its rebooting constantly just switch it off and plug in mains charger. It will trickle charge over time.
2:Take out battery and connect the usb cable to both phone and pc.
3:Hold down the U key and then insert battery hopefully this will put it into update mode before it hits the reboot cycle.

Hope this helps. By the way do you know what started the reboot cycle, programmes installed/updated ect.
Im pretty sure miy problems started when I was messing with the SunJava scripts.