It is not possible to check it ATM, I have returned GTX 280 to a store such as there is no difference and I can't move 9800GX2 to a middle slot - it is water cooled and the length of tubes is minimal. But I understand your point - you want to check if the first slot is a cause, therefore I will remove water cooling and check the card in a middle slot. It will take some time though.
Thanks.