Oops! I guess it is called "Clear Recent History..." under Tools now. You will want to click the down arrow at the bottom of that window and make sure cache is the selected box. You may want to uncheck the other boxes if you do not want that data cleared. Make sure the drop down menu has Everything selected. Try that and see if it helps.
It sounds like the download for the high bandwidth clips is getting interrupted at some point before it finishes downloading since the low bandwidth seems to come through fine. Did Internet Explorer have the same issue before you switched to Firefox?
Paper Wishes Web Team