Sounds as if you have a really bad "karaoke" mic. I would try a different mic. It doesn't have to be expensive. Behringer sells one for about $30. that works very well.
I generally use Shure SM58, if I'm plugging directly into the keyboard. I use Shure condenser mic if I use a mixer with phantom power. I have also used the Behringer, and it is quite close to the SM58.
DonM