We have had this fault on 3 projectors that we support & we have found it is due to colour wheel irregular speed . We have found that removing the upper case & cleaning the dust out of the unit including the air vents cures the problem. be carefull around the colour wheel as it will break very easy as is made of thin glass. if youdo not have the skills to dissmantel the unit then contact a projector servicing site & ask for a deep clean . hope this is some help.