DST800 Locking Up



:? My depth readings lock up to random values between 10 ft and 25 ft. I use a DST-800 transducer and display on a MFD12, an FI-503, an FI-50 "MULTI", and MaxSea software. Even without the NavNet turned on, using only the FI-50x displays, the values lock up. If I cycle the power to the instruments, the readings will someties return to valid values, but often not. It seems to be much worse when in shallow <10 ft water. The problem is geting progessivley worse and began about a month ago. Any and all input in resolving this problem is welcome.
Is the DST800 keel mark pointed correctly to the boats keel?
Is the unit pointed straight down or do you have it mounted on an angle?
If mounted at an angle, what angle is it mounted at?
Does the problem happen while the boat is not moving or only when moving?