ST60 Depth Issue

[NeilB]

Recently removed a RL70C and replaced it with a C120MFD. Employed a qualified technician to install and run all necessary wiring. Connected new (used) C120 to my existing ST60 Wind, Speed & Depth, and ST6001+ Auto Pilot with SG1 Controller Computer. I had the tech connect the old RL70C at Nav station as a slave.
The problem is after the install, about 1 1/2 hours into each cruise trip,
the ST60 Depth starts doubling and tripling the depth. After maybe 30 minutes or less, it either shows ------ across or freezes with "last depth" which was incorrect anyway. After being turned off overnight it starts out each morning working properly until underway 1 1/2 to 2 hours and it will then register 99 in 25 ft or 38 in 17. All over the place. I tried leaving it on just sitting in marina and sure
enough about 1 1/2 hours of being on it went from 2 ft which was correct as it was low tide in the marina to 17, 18, 20.
Any suggestions as to possible problem, Wiring? Computer issue?
I am at a loss. If it matters, it is a 2004 Catalina 36MKII.
Thanks!
NeilB