I experienced the same situation and then the depth would jump around, lie and flash on and off at the same (wrong) depth. Almost went nuts trying everything. Finally tried a new transducer and it's performing great. Will have two transducers on all trips from now on.
Took along my old Eagle Fisheasy2T on fly-in in August as a backup. X-15 worked fine for most of first day and then acted up on me. When getting ready to hook up the Eagle I noticed that the Eagle transducer would fit my X-15. Not the same transducer, but it works great. Gives the temperature also. The fix was so simple and so easy, but about $75. I still love my X-15.