Hey Glenn,
I dont have the same unit you have, but mine is the Lowrance H2Oc with Navionics Gold. I find if I hit zoom too many times quickly, the system takes a long time to catch up. The result is a blank screen until it can fill in all the detail. It can take well over a minute at times. If I zoom in once let the map draw, then zoom again, let it draw etc, it works just fine. Not sure if it's the same issue