1973 23' cc with bracket performance??

[BigLew]

If your cavitation plate is "on" the water while on plane, then I would bet you have a partial obstruction in you cooling system or a bad water pump/impeller. It sounds like when you need all the cooling you can get is when you have problems and the most significant contributor to cooling capacity is flow volume. Something is retarding your flow volume from achieving max flow. It could also be a bad/wrong thermastat; either not opening enough or something. Depending on the history of the engine, you might want to flush it with a salt/corrosion removal product/solution. If if hasn't been flushed in some time or stored open to entry of bugs, mice, etc., something could have gotten in and made themselves at home and something still in in hte system restricting the flow.

Just my 2 cents.