Hey girls...this is kind of an old question, but I just came across it while searching for something else. I don't know if any of you are still having this problem or not, but if so, here is my input. :) I used to have this exact same problem (I also use PSE5) & I did this very simple thing to fix it.
So once my LO is done I save it as a PSD file at the full size. Once I have that original saved I choose "merge visible" to merge all the layers together. Then I just change the image size to 600x600 & the shadows look normal. This has solved my problem. I think the weird drop shadows must have had something to do with the layers not being merged. Then once I have saved that smaller image for posting to the galleries, you can always just choose edit & undo until it is back to its original size & the layers are no longer merged together.