My phone was behaving erratically over the last few weeks (Random reboots, frequent freezes etc). After countless hard resets, it became worse that I couldn't even complete the setup after the hard reset. The phone freezes as soon as it gets connected to WiFi or HSPA networks. Samsung service center returned the phone as fine twice (Express 3 hours Service) but the problem persisted. I demanded to meet with their manager and proved the senior technical engineer wrong right there in the service center. They took 45 minutes to replace my motherboard after all the fuzz that I made and so far this replacement motherboard is working fine for more than a week now without a single reboot or hang.
Strangely enough, my friend's phone with the same affected eMMC hasn't experienced a single reboot or freezing issue.