The ALC controls the power and this voltage will change due to a number of reasons.
The set power from the CPU, SWR, temperature, current etc.
Take a look at the ALC line voltage to see what this voltage is doing as the power changes in the time frame you are interested in.
You have POVC, FORL, AMS, ICH, ICL, IDS, RELF, EALC, UNUSB, and ALCL that all affect power by changing the ALC voltage.y
Investigating this area for a voltage that changes with the power output should aim you toward the problem area(s).
If the ALC voltage does not change with the power output then it is likely that the problem could be somewhere else, but I would first rule out this area as it has direct causation on the power output of the radio.