LCD overhead projector

Having read the guide on how to make a movie projector out of an lcd screen and an overhead projector, a friend and I decided to build one ourselves in the past week.
Everything has gone perfectly to plan, and we get an image up on the wall. However, the LCD panel continuously changes back and forth between the normal colours to the inverse of the normal colours, on a regular cycle.
I would like to know if anyone could give us a clue as to why this occurs, and how it may be fixed.
Thanks.