Jumper wire burned

[vulcrider]

I have a 91 DT25C. Just this summer, I noticed smoke from the lower end and some from the tiller handle pivot. I discovered the jumper wires that connect the clamp to the lower housing burned and melted plus both throttle cables melted to the point that one was fused to the conduit.
I found the one of the cables had rubbed against the lower engine housing, probably causing a direct path to ground. I replaced the cables (insulating them from the housing) and the jumper wire. Test ran the motor and hooked an ohm meter across the jumper wire screws with no indication of current.
Today, running the motor wide open as well as long idle periods (large no wake zones) I found one of the jumper wires burned and melted again. No problems with the throttle cables. A simple test in water showed no current flow at one end of the jumper wire. No other issues noted.
The motor is mounted on a 16' Starcraft fishing boat. It has no wiring grounded to the boat. All wiring is isolated (lights, battery, trolling motor) and I found no indication of current flow from that segment. The battery checks out fine.
Any thoughts as to where the current flow is coming from? All engine grounds are clean and tight. The battery cables to the engine are new and tested for shorts, all OK.