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Are you using the "BL" or the "AZ" mode. I use the "AZ" mode I and I haven't seen mine do that. I've had it since 2006. I'm going out today again and I will pay more attention to it.
When I'm hole hopping I leave it off until I mark a fish and then hit the zoom. Sometimes it will work just leaving it in the zoom mode and hopping, sometimes not and sometimes it will fatten the bottom a couple of feet. Not sure if it has to do with the bottom make up or not but thats my guess. Another problem with the unit is fishing sharp breaks it will fatten the bottom reading as well. One trick is to turn the unit to normal, increase the gain to around 8, flip it back to AZ and then back the gain down.