Check that you are not overdriving the mixer (turn down the CD/Mic/etc volume controls and see if the problem goes away).
Don’t yell into the mic – and if you must, turn the mic volume down. Tip: keep the mic close to your mouth; don’t speak into it from a foot away. 1-2″ is ideal.
Check that the amplifier inputs are not being overdriven by the mixer – look at the clip lights on the amp – if they are staying lit for more than a brief flash occasionally, you are overdriving your amp and endangering your PA. Turn the mixer down until the clip lights only flash occasionally.