Iphone 3g keeps rebooting itself - Help!

Since yesterday my iphone 3g has been going very slow. Today it decided just to give up on me completely. Wouldn't let me into any of my apps etc. So connected to my laptop, but gave me two error messages the first one being a number saying it was an error. The second one said that the iphone wouldn't let it connect.

It has now decided to just keep rebooting itself, taking about 5 minutes to load up & then flicks to home screen then reboots again. Tried holding in the off button & home at same time to over write the restart. Nothing.

Any suggestions would be grateful? Phone is just 2 years old!

Thanks x