Yeah, they're both in the same machine, the only difference apart from the actual cards is which PCIe slot they're in, obviously. I do occasionally get HW errors on the HD 5830 (about 1 every 2-3 days), but I don't see the connection.
Strange, it's hard to imagine that any minor difference between the cards design would affect its ability to get the share from the card to the pool that significantly.
Out of curiosity, is the 5830 also serving your monitor?