Hello bonoes. My family has 4 arrives which I have had since 03/20/2011 and not one of us experience this problem. Is it possible it is an isolated incedent for your phone specifically? Till this day, I have yet to ever have a hard shut down. How long have you had it? Have you tried resetting it to factory setting? To reset go to settings, about, reset your phone (towards the bottom). Maybe Sprint need to re-configure your phone to your line as well.
bonoes77, if you do try a hard reset as advised by Venom01x and the issue persists, I would suggest having your device diagnosed at one of our service and repair locations. As long as there is no physical or liquid damage to the device, they can check it out and determine if repair or replacement is needed under warranty. You can use the store locator to find the closest store to you http://storelocator.sprint.com/locator/. Be sure to click the repair button to make sure the store has technicians to service your phone. If the warranty has expired or if you don't have TEP you will be required to pay $35.00. Hope this helps and thank you for your business.