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Overheating 3126B

I have a 35' Cabo with 2 x 3126B CAT engines that we use for our Fishing Charter business.  Both engines were rebuilt a little over 12 months ago.  Recenlty one of the engines recently started overheating (205F) anytime the engine was operated over 1800rpm.  Under 1800rmp it seemed to work fine at around 185F.  We are in Central America where the intake water (salt water) is 84F.  After experiencing the overheating we changed out the Fresh Water pump, Salt Water Pump, Thermostat (175-182F), verified the risers were clear, cleaned the exchanger, verified the intakes on the bottom of the boat were clear. After all this work the engine continued to overheat.  During sea trials we noticed small bubbles in the freshwater coolant tank.  Immediately we suspected a headgasket and removed the head to replace the gasket.  Before resurfacing the head the machine shop ran some test and informed us there was a small crack.  Rather than try to repair we opted to replace.  $3500 later for a ReCon head we had a new rebuilt head on the engine and were off fishing again.  One day into it the problem of overheating (202F) presented itself again.  The boat captain loosened one of the small hose fittings on the engine that goes to the overflow tank and air seemed be released.  The temprature dropped back down to 185F.  We ran the boat for +1 hour at 2400rpms and the temp stayed stable  I am now wondering where to go from here.......?  Any thougts would be much appreciated.