Have you noticed any loading stalls (or irregular freezing) when you play Wii Sports?
- Especially when playing the training games?
Any regular/irregular Disc Read Errors?
What about the game disc itself?
(I doubt you've managed to scratch it to the point of error this early in it's lifespan but it's worth looking at to cancel the possibility out.)
If all the answers are NO then I recommend exchanging the disc to confirm that you didn't receive a bad disc, however if you answered YES to the first three than I recommend calling Nintendo and relay your exact problems with them along with the questions to answered yes to.