When my engine overheated, the wiring harness at the engine literaly melted in spots. The mechanic had to inspect the entire harness around the engine. I still get some fluctuations when i'm running the lake. If you have the time and the wiring schematic, follow the wiring harness from the dash to the engine compartment. There might be some connections that are butt connected that corroded and have intermittant connections. I guarantee you'll know alot more about your boat when you finish.
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Alexj
1995 Four Winns Horizon 190
OMC 5.0 RL