Can't offer any help, only sympathy. My 7si does the same on LM. It's worse on Navionics but works fine using Basemap. Also, when switching to chart view or a combo/chart it will periodically just shut down. Unit went back to HB and came back clean, no problem found.
My old Garmin had fast, smooth transitions using their basic lakevu maps. Upgraded to HB to get more map options. Have more options now but unit is less functional.