I have a 917c mounted in the bow of my Ranger bassboat.  The transducer is enclosed in the bottom of the MK 101 trolling motor.  About a month ago I turned on the unit and a verticle black line about 3/4" thick appeared on the screen.  About a week later a second verticle black line just to the left of the first appeared and the next week a thrid black line appeared just to the left of the second.  I now have three verticle black lines, each about 3/4" thick.  I ran a diagonostics check and everything came-up OK.  
Any ideas?
Thanks,
BassinBill
Chapin, SC