For solving earthing/groundign related problems for audio and video signals read my document "Ground loop problems and how to get rid of them" available freely on-line at
http://www.epanorama.net/documents/groundloop/index.html

Generally using a separate power feed for the whole video system and the lighhting dimemr system is a good idea. When using separate video power, yse the same power source for both the video production location and for the video projectors. And keep decent distance between video cables and the cabling related to dimming system (dimmer power feed, wiring to lighting instruments etc..). Try to get one meter or more clearance (if that is hard try to have at least half meter).

When those do not help and are no practical, there are also video line isolators and hum bugging transformers that can used to solve various video noise problems caused by ground potential differences and dimmer noise. Those devices (some of them manufactured by me) have worked well in solving problems in various application from rock concert video projection to TV broadcasting systems.