Nokia :: 5800 Auto Reboots Every Two Hours And Hangs Often

I bought a new Nokia 5800 express music less than a week ago. Right from day one I got problems like auto reboot and mobile hang often. My phone came with software v 40.0.0.005. I took it to the showroom where I bought this phone and they advised me to update latest software and that will solve the issue. I updated to v 50.0.005. But within another one hour I ended up with same problem. I am seeing pink horizontal lines before reboot. It automatically reboots while messaging and when I try to do something with maps. It hangs often when I see any video file. I took it to nokia care again but they formatted my mobile and installed v 50.0.005 but am having the same problem again. How do I solve this issue.Is there any option to claim for a refund?? Any option to replace handset?? Preferably different model? I find lot of problem in using this phone which is not user friendly.