Why does my voice do this when I sing!?

The same thing happens to me. I can sing high notes cold without any warm-up, however, after singing for about a half-hour my high notes sound like screeches. If I stop singing for about an hour or so, then I can again sing the high notes. What all this means is that the high notes are not truly within your vocal range, so those notes are more of a hit-or-miss. This hit-and-miss of the high notes happens to professional singers as well. I suggest that you either skip the high notes completely, or simply transpose the music downwards so that the music's highest notes are always within your vocal range.