@Manish: does it happen when you change the channel or when you switch on the TV for first time. Try disconnecting the HDMI and reconnecting. Also, as mentioned by Jerome above, try connecting any other device with the same HDMI cable and to the same port. If the issue is not there with any other device, STB is the culprit here.
@Nick: OP has a problem with the box and not complaining about the service. Let's help him fix the problem and later you can ask him to switch to TataSky or whomever you like. Thanks for your understanding in advance.