Help on the power voltage and PS Premier question

I know I have some problem with my voltage at my home as recently I kept hearing breaking noise from my speaker but all my gears run well in other places. So I borrowed a PS premier to test the voltage. It turns out that the input voltage shows slightly lower than average, 113-4, but the amazing part is during the night, the output voltage only shows 90! and the output THD is jumping bettwen 10-10.5, since PS audio does not suggest the ideal output THD range, does anyone know what does this data means and why the output voltage will drop so much? supposedly PS Premier should keep the output voltage at 120? in the day time it back to normal range 112-115.

I do not live in a house, I lived in an apartment in the city.
Are you toggling through the various modes on the Power Plant Premier to view the input and output voltage and the input and output THD?

The output voltage should be 120-121 and rock steady. The input voltage can vary depending on the load of the electrical grid in your area and apartment complex which I presume would be more at night when people are home.

On mine, it also shows the difference between the voltage output in and out if you press the mode button. The THD out on mine is .2-.4 depending on the time of day. THD in can be as high as 3.5%.

The PPP will default to connecting directly to the output oulet's voltage and THD if it is overheated, the draw on it exceeds its capacity of 1500 volts over a continous period or is faulty.

Also, look at the vents below the unit. Are they rectangular or oval? The newer and improved units have oval air inlet vents.
Nick, thanks for the information. I did toggled through all the models on the unit so I can get these figures. I think there are problems with the power supply quality of my apartment. I know the PS can default conneting to the wall outlet, but I think the wall outlet is where the problem comes from. My question is, what is the reason causing the high output THD%? is that because of the noise signal from the AC or the surge? how can explain to the power company and ask them to correct the problem? I can not just tell them there are noisy from my speaker, they will think I am nuts.