My Reverse Rear camera stops working then starts again

what do I mean by this ....

well I start the car, put it in reverse and the screen displays the text "Check Entire Surroundings"

but it displays either a green line horizontally across the screen usually half way up or near the bottom, or just a black screen.

once it displayed a static multicolour snow like screen, like those pictures you get where you look at it for a long time and eventually you see an embedded picture... yeah like that.

then after a while, usually after turning off the engine leaving the car to go shopping then back in, turn the car on, put it in reverse all of a sudden the camera is working again.

details you might need to know:
it's a 2008 new shape
it's an automatic
i have had the MyGig headunit software updated (it was happening before I had it updated).

any ideas?