Something strange! Issue seems to have resolved. Technician visited and I could reproduce this issue. But he changed resolution back to 1080i from 1080p (I had set it that way). Since then, I am not able to reproduce this. As per technician, this is a known issue for some STBs and will get resolved with firmware updates. He has advised to keep resolution to 1080i only. I really hope I don't face this again!