I've heard of lamp flicker before. Nobody seems to be able to say why it occurs exactly, but if the projector is under warranty, then I would call Mits about it.
I will add, 1,500 hours may be under the 'normal' amount listed, but is very typical for a lamp to end its useful life. I generally recommend that people budget for a new lamp at 1,000 hours. Lamps dim significantly over time and at any point after 1,000 hours people often find that the quality of the image suffers significantly due to this dimming.