Xbee Arduino Fio Analog pin results

I have a microphone with amplifier connected to pin A0 on
my Arduino fio. When I use the processing graph sketch example, I get an almost straight line for the data coming from my microphone ,that gives amplitude when I start talking. (amplifier connected to 3.3 v, GND and A0). This works fine. When I do nothing else than plugging in the xBee shield on the fio board the graph showes holes like 2 times a second. Here is a link to show the graph.

http://dl.dropbox.com/u/8308826/testplot.JPG

Anybody any ideas? I am new to xBee so it is probably a beginner problem.

Considering that the FIO is not shield compatible, this statement makes no sense:

When I do nothing else than plugging in the xBee shield on the fio board

Unless you found a XBee shield for the FIO, that is. If that is the case, you should post a link to it.