Sautered Soldered my connections and never had the problem again.
Soldered connections (without other mechanical constraint) aren't even allowed by ABYC standards. A crimped terminal connections is actually better.
The OP needs to look at the clues. The bow unit works fine when floating around, but starts getting flakey when he's moving on plane. About 99 out of 100 times, that says there is a connection issue that is affected by the movement and vibration of the boat on the water. When an outboard is running at RPM above idle, the boat's electrical system voltage is at its highest because the charging system has it jacked up. What he almost certainly has is a continuity problem (loose/bad connection or broken wire), not a "voltage" problem.
How to find the offending problem? Park the boat, run the bow unit, gain access to the wiring, start moving and wiggling the wire harness at the bow. When you get to a problem area, you'll cause the unit to do like it did on the water. Sometimes it's a crimp where the power is tapped that's not tight, as an example. Sometimes a wire that can flop in rough conditions that has moved back and forth so much that the strands of wire are broken but still touching inside the insulation, though not as common.
One last possibility: Have you considered that the HDS9 has an issue with an internal loose connection? It could be that the G forces from vibration when you run across the lake are causing the unit itself to have an issue. Do you have access to a different HDS 9 to try on your bow? An alternative is to take yours out of the bracket and bump it on the deck while running to see if it flakes out. Don't slam it, just give it some bumps like it might get out on the lake.