Here is another suggestion, you can make sure that you connect the shield on your mic cable to the chassis of the mixer. Here is additional information on what is called the pin one problem
http://www.tvtechnology.com/pages/s.0071/t.1589.html
If you have access you need to also look at the entire cable path from the mic to the mixer, I am guessing there are various plug in boxes along the way, and these can be problematic. Get your self a good quality mic
xlr cable, and see if plugging in the mic to the cable and the cable directly to the mixer fixes things. Look at the power plug on the end of the mixer and see if it is three pin and if all the pins are intact
Sharyn