Multimeter not working when metal halide ballast plugged in.

Hello- I have a metal halide ballast that when plugged in causes my multimeters to give nonsense readings. When plugged into a wall outlet, the meters are giving the expected result of 120 volt ac as long as the ballast is not plugged in. When I plug in the ballast (different outlet) the behavior differs between the two meters. My 7 function unit gives huge readings that are all over the place. Usually somewhere between 200-500 volts ac. My 14 function meter switches to dc and gives readings in milivolts. I tried plugging the ballast into a different circuit than the one I tested and got the same result. What is going on here???