Most of the times this is due to a poorly built, cheap knock off lamp, sadly the market is saturated with them...
I would first verify its authenticity, it might not be shinning in the correct spot due to a manufacturing imperfection. Mitsubishi uses Osram brand lamps, but everyone in the business recommends Philips because they last longer and are brighter. <<< if you do not have an OEM lamp I suggest you try one first.
Sometimes, if your lamp is an original Osram or even a Philips, you can still get this band if:
1- the lamp is not installed properly inside its enclosure or the enclosure is not sitting flush in the TVs lamp tunnel.
2- the optical block might be misaligned, if the TV was moved around alot during replacement or transport, the Light engine might need some calibration.