your amp draws 1440 max watts that is close to the max a regular 15 amp circuit can handle. 12amp's when the power is 120v more as it dips. so a 20amp circuit could be beneficial. i'm guessing you don't run it full volume though... that said you should get your line voltage checked.
as cleeds and Kenny said whats your utility giving you?
i'm not sure what your using to check the line voltage but check it at your outlet and the utility's entrance to your houses panel they should be the same or very close (less then a volt with your amp off). If they are both dipped then its a utility issue. if not its a house wiring issue