Diagnosing Problems with a PCI/PCIe Card
Firstly, the good news: assuming there are no manufacturing defects - and the correct installation procedure was followed - then expansion card problems are rare. If a card has been operating without problem for a while, then it could be that it has become damaged by overheating or by a faulty PSU or a power surge.
Cards normally have a defined purpose - such as graphics, SCSI or networking - so any problems in it's area of expertise should start to ring alarm bells concerning that card. If the functionality of the card is available on the motherboard as well (-e.g. graphics, networking), then the easiest way to confirm that it is the card that is at fault is to remove it and see if the problem goes away.
If the functionality of the card is available on the motherboard as well .. then the easiest way to confirm that it is the card that's at fault is to remove it and see if the problem goes away
Obviously, if the card is the only source of this functionality (e.g. RAID) then removing the card will remove that functionality altogether but, if you suspect the card is causing problems elsewhere in the system, it might still be worth doing this in order to see if these side-effects are also removed with the card.
Before you assume a card is faulty, try moving it to a different slot on the motherboard (-assuming there is one). If you have several slots, but they are all occupied, try swapping the cards over in the slots to see if it is a particular slot at fault, rather than the card.
There is normally only one course of action (-assuming the card is still required) if a card is faulty: to replace it.