Downward drop in power is one sign of over modulation. This is caused by 'Flat topping" or limiting of the signal.
Whistling is about the worst way to test a transmitter.
Looking at page 33 in the FT-990 manual:
-RF Carrier is limited to 25w max. You may set a lower power if desired.
-Modulation is controlled with the MIC control.
-While speaking adjust MIC gain so the Po Meter just moves (on peaks-my comment). Setting any higher will result in over modulation.
AM modulation and Linearity is best measured with an RF Sampler installed inline after the transmitter (and can be moved to after any amplifier for checking it as well). The rectified output is then fed to the X,Y plates of an Oscilloscope where a triangular pattern in is formed. A standard way to evaluate and adjust the radio is to input a tone, e.g. 1000khz into the MIC input.
An RF Spectrum Analyzer is the best way to check for resulting "splatter" that over modulation causes.
Clean RF Inc, has some simple tutorials.http://www.cleanrf.com/applications.html#app_2