Using Netstumbler: Losing signal intermittently...

Been doing a bit of war-drivin and seem to be able to pick some quite a few AP, but just wanted to know why I seem to get a intermit signal.. I get good signal, with a little line noise and this lasts for say 20-30secs and then a purple line appears and I lose connection.

After another 30 secs it does the process all over again. I have the screen shots if anyone wants to see them to get a better picture. Can anyone help me here?? Cheers

FYI: I am using a Tosh Tecra 9100 running XP Pro, Arinoco Card and panel antenna...