I have this weird thing in Photoshop (CS5): When I'm zoomed in at 100% my lines are really smooth, but as soon I'm under 100% or over 100% they look weird, like this:
This is how they look at 100%:
Nothing to worry. Your computer is just trying to make a quick preview on the zoomed image. Nothing is happening yo your file.
Sometimes a program makes some calculations to smooth an image on the preview, sometimes makes no calculations and just drops some pixels, and some times makes a lot of calculations so you can see the image smooth, doing something called anti alias, which is averaging the values of adjacent pixels.
But that is just viewing your image. Zooming in and out does nothing to your image, as I sayed, it is only the preview.
Working on Photoshop normally implies that you work at 100% or at a round number like 200%, 300% etc. This gives the computer enough information to make a proper preview on what is really happening on the file.
Not rounded numbers, like 73.33% or 127.73% makes the computer decide what to display and what to average, and it is not a precise way to work with raster images.
Oh. If you decide that the correct zoom to view your image is 73.33% as your example shows, you might consider resampling it with let us say a bicubic or bicubic sharper method. This will change your image forever, but with the proper calculations to smooth the image.