[Chris Hibner]

The other people have said it in a round about kind of way, but I'll clarify it here:

If you are using a gyro:

DO NOT move the robot after power on until you enable it (it really needs a few seconds of zero motion, but I'd wait until you enable it to be safe).

After power up, there is a period of time when the CPU is determining what the voltage is of a non-moving gyro. Let's say that during this period you move the robot 2 degrees, and this "gyro-zeroing" period lasts 2 seconds. That means you've moved the robot an average of 1 deg/sec. When the zeroing routine is finished and the robot is sitting still, it thinks it's spinning at -1 deg/sec (since 1 deg/sec is what it thought was "still). Therefore your gyro reading will be changing at -1 deg/s

So, be absolutely sure that once you turn on the power, your robot does not move at all.