Might try the head temp sensor.
may be sending a faulty signal to ECM when engine warms up.
The various sensors all tell the Ecm how to manage the fuel,
air and timing. thus the end horsepower/ torque output.
Your complaint is it drops AFTER several runs and engine is heated
up.
Another thing is how warm is the air coming in in the dyno room?
Warmer air and power will drop somewhat.
Like when you go riding on a cool day engine seems peppier than on a hot dry day.
Might also check the air intake temp sensor if yours has one.
They may not be far enough out of spec to set a code, but just enough to change the afr and timing