Hi,
Devices inside your pc share system resources. Sometimes these devices do not play well with each other. Sometimes the things on the motherboard itself conflict with cards that you put into certain pci slots. When this happens if you move the card to another pci slot it will share other resources that do work well with the device. The thing is when the card will show up in device manager as being "no conflicts," but still under demanding situations present anomalies like you describe. Changing the pci slot the card is in will change what devices it shares irq's etc. with and work better.
I remember the infamous via and sblive problem on some kt133 chipsets. I also remember my older board used to have problems unless the sblive was in pci slot 3 or 5. People would put thier sound cards in thse slots and it would work great.
If you look at your motherboard the pci slots are the white slots towards the lower rear of the case.
Thats the best I can explain it at 1:00am anyways.
Will
EDIT: Grammer