Mine started doing that about two weeks ago, my garage suggested removing the car battery for 5 mins and reconencting it [make sure you have the unlock code for the CN+ first!]. However, I didn't get round to doing this, as one day last week on the way into work it stoped resetting and flashed up the "SIM card not inserted" message (I had removed my card about 3 days earlier), I pressed 'esc' and it filled in the background of the phone screen. About 40 seconds later it slowly filled in the text etc. and started to play music!! Its been fine now for the last week!
I have no idea what caused it, or what fixed it, however I was playing with the light sensor at the time to try to get it to dim the screen so it wasn't quite so annoying!