Hey Amanda, yes it is a common problem with Marketo's preview pane so you are not alone. Marketo's preview pane is not a true view of what your email will look like across all the various browser and platform combinations you will be sending to, so it is best to use an outside Email testing solution (EmailOnAcid and Litmus are the 2 main ones) instead.
Like Jennifer, I too use EmailOnAcid (EOA). With it you simply send a sample email from Marketo to your EmailOnAcid autoprocess address and the next time you log in to EOA you will see the results in your dashboard. That is just a visual though EOA can't help you solve the problem. Solving the problem is another matter entirely! I posted a couple blogs on that topic in the Houston User Group if you are interested.