2D sprite texturing problem

I'm working on a little low-res sprite game using OpenGL. I'm having problems with texture artifacts- at first I thought they were due to scaling, but that doesn't seem to be the case. Rather, it's due to float coordinates. It seems that whenever a sprite's coordinates are near x.5 on either axis, it flips out. In this case, the textures are 16x16 and the resolution is 320x240. Good: Bad: Any ideas?