i50 Tridata erratic speed readings

[jumby]

i50 Tridata with depth, speed (ST800-P120) and wind transducers connected installed below deck at nav station. Individual wind, depth, speed displays networked and set as repeaters in cockpit. Depth and wind functioning properly. Repeaters functioning properly.

Speed readings are erratic. Motoring at 4-5 knots the speed reading will jump to 8 knots then back to 4, then jump to 14 knots then back to 4. At times, sitting dockside it has read 1-1.5 knots with no current present at slack tide.

Done to date: At suggestion of Raymarine tech support have done multiple speed calibration runs over a measured nautical mile to no avail. Also as suggested by support checked the transducer paddle wheel with a compass to ensure correct polarity of magnets. Have done a dockside calibration for speed offset. Have replaced display. Have replaced transducer. All to no avail as the unit still has erratic readings.

This is installed on a very basic Island Packet 32. The only 110v systems on the boat are a water heater, battery charger and outlets. While motoring no 110v is present. While motoring the only current present is the alternator charging batteries. When reading speed at dockside there is no shore power present, no charging going on. The wiring and transducer runs are following the previous installation of instruments (autohelm) that were in use for at least 15 ish years with no such problems. When replacing the transducer I even installed it into the thru hull fitting and ran the transducer wire directly to the tridata from the bilge, through the cabin bypassing any and all existing wire runs with no difference. I ran it to the speed display in the cockpit and made it the master with no difference.

At this point I have no idea what to check or do to resolve this. From raymarine what I am getting is send everything back to them for service. I have a hard time believing two sets of displays and transducers are at fault. I'm not past thinking I have somehow missed something that is causing this but feel fairly certain the integrity of the install is pretty good.

Little help?