Garmin Fish Finders Goes Wacky!

[saltfisher]

I have a garmin 740 i think is the model number. A few years old. Sometimes it goes on the fritz and shows 600-900 feet of water and tons of fish. Sometimes i can turn the unit off and back on and its ok. Othertimes it keeps doing this. Any clue what the cause could be? I was thinking bad connection because sometimes it turns off on me and have to turn it back on.

Thanks