Why can't I move a selection one pixel up?

I'm new to Photoshop (using 7.0 in Windows 2000). I'm trying to draw a spaceship. Here's my file (14K):

http://www.people.fas.harvard.edu/~night/Stuff/hepta.psd

I'm having the strangest problem. The layers are not aligned how I want them to be. I want Layer 8 (with the cyan) to be moved up exactly one pixel, but I can't do it. With the Move Tool, I can move it in every other direction one pixel at a time, but when I try to move up, it jumps two pixels at once. This applies no matter how I make the selection, it seems.

Thanks in advance,
Christopher

I want Layer 8 (with the cyan) to be moved up exactly one pixel, but I can't do it. With the Move Tool, I can move it in every other direction one pixel at a time, but when I try to move up, it jumps two pixels at once.

Try using the up arrow key, that should move it exactly one pixel (hold down shift and use the same key to move it 10 pixels).

How simply that worked! Thank you very much!