[Medicone]

What temps are you seeing?  As jclark003 mentioned, get a IR gun...it will save you lots of time in diagnosing the issue.  Get the temps on the elbows/risers, heat exchanger, thermostat housing, portions of the engine where the temp sensors are, etc.

Did you change your circulation pump or just the raw water pump?  The circulation pumps can deteriorate as well.

Best course of action would be to get the temps of the elbows/risers to figure out if it is a raw water issue or a fresh water issue.

If temps indicate raw water issue, cheapest thing to do would be:
-check the heat exchanger for debris - take off the inspection caps and spray water through the tubes.  You may find some old pieces of the pencil zincs, impeller fins, shells, etc.  When was the last time the elbows/risers were changed?
-check the seacocks
-check the sea strainers for debris
-check the seal on the sea strainer and the hose connections for air leaks

What do you mean when you say the system was "flushed"?  Was the coolant flushed or did you flush the raw water system?  Was the raw water system back-flushed such that any debris would come out of the oil/trans cooler(s).  Did you run rydlyme through it to descale?

If a fresh water system problem:
-did you put the right thermostat in?  I'm not sure if the 7.4 is a 170 or a 180, but check the manual.  Is the thermostat correctly oriented?
-inspect the circulation pumps
-what is the condition of the coolant?  when was it last changed?

Had you not said that it idles hot then cools down with load, I would have asked whether you:
-burped the system after the coolant flush, 
-used the correct coolant (dexcool 50/50 mixtures)
-verified the seacocks were open and clear of debris
,but that likely wouldn't be the problem if it cools down with a load applied, at least to my knowledge.

Good luck!
-M1