Merc 7.4 rising temperature

Hi. I'll try and be brief.  I have a 7.4L merc, Bravo one raw water
cooled, moored in shallow salt water (gross I know).  I have a new
impeller in my water pump which I change every season.  I also have a
new thermostat housing I installed just before going in the water.
The old one had some corrosion I didn't like. It's 16 years old.  At
idle my temp hits 160 and stays. I've run her 4 times so far.
During my first run after dropping her in the water at about 3000 rpm
my temp started creeping up to 170.  This has never happened so I
backed down and temp came down to "normal".  My second and third time
out the temp stayed at 160 through all speeds.  Yesterday  I ran her
and the temp was creeping up over 170 again.  Starboard side manifold
was hotter than ****t.   The fact it's been inconsistent has me
thinking or hoping it could just be my thermostat not opening properly
but why would temp be normal at idle.   Any thoughts on what would
cause rising temp under load one day and not another?  I'm not going
to be able to get back out the boat until this weekend but this is
eating me up and wonder if anyone had any thoughts.  Thank you.