New Purchase Order Shows Items from Old PO

I have seen this 3 or 4 times now on our system.  When I create a new purchase order and select an item, I click "add" so that I can accept that item and go down one line to select a new item, but after clicking "add" the screen flashes and then shows what appears to be a randomly selected, old PO from our system.  There are all of a sudden various items on the PO that I did not select.  It does not matter if I use Firefox or IE.  Has anyone else seen this?  I wish I could