Blitting Problems

This is what I have. A primary surface with a buffer attached. A offscreen plain surface. I have clippers on the offscreen surface and the buffer. Both offscreen and buffer are 640x480. I draw my map onto the offscreen surface. Blit that to the buffer and flip to the primary surface. When you press up, down, right, and left keys it moves the offscreen surface(map) around on the buffer. If the x and y are within the buffer everything is fine. (I clear the buffer with a green color so I can see the difference between the map surface and the buffer surface.) The problem comes when the x and y of the map surface is negative (off the buffer surface). If the y is negative, the map seems bigger than 480 and has junk in it. If the x is negative then the map is left on the buffer (I never see the green buffer behind it.) Does anyone have any idea why this is happening? -Rick