So I'm not exactly sure what you are seeing/describing, but when I pasted this into my own prizes cell I did see this weird rendering glitch where the bottom of the marquee kind of "smeared" and didn't update properly until the rest of the layout updated (every one second). I moved the cell and then removed the "height" attribute, but it didn't change. So I wrapped it in a div to control the height instead and the glitch was resolved:
<div style="height: 200px"><marquee direction="up" scrollamount="1" scrolldelay="50">
<prizes>
</marquee>
</div>
I don't know if this will help but give it a try I suppose.