That is really weird, unfortunately I can't fully help you because I've never heard of anything like this happen before.
Have you put anything on an SD card and then put it on the wii? Have you manipulated the hardware or software through an SD card in any manner? If you have, then it's probably the wii that's acting up.
And if you didn't do anything to the wii at all....Then it's probably the wii's fault anyway. I really don't know how to help you, just call Nintendo and get a replacement wii, providing you're still under warranty.