When tracking down a problem affecting a computer, try to isolate areas of the computer thus narrowing down the diagnostic tests you need to do. Also if the computer can boot up to the operating system then use a third-party tool like a burn-in test to check the system out. If you need to check the internal hardware, it is best to remove all cards (except the graphics card) and drive cables. This will then identify that the problem is the motherboard, memory, CPU or graphics card if the fault still is present.
If an electronic part is faulty, the fault usually appears soon after it has been installed. By contrast parts that have been in a running computer for months are likely to continue to work as long as nothing changes in the computer. Over time electronic parts do wear out and adding a new card could put a strain on the other components as the new part may draw on more power than can be supplied either by the power supply or the motherboard.
Most commercially built computers are run continually for over 24 hours to test for faulty components. This test is called the burn-in test.
The majority of computer faults are not actual faults but problems cause by the user. A simple thing like moving the system to dust underneath can result in a cable becoming disconnected and creating a problem.
You may think that you have found the fault but before you discard any part, test it in another computer to confirm your diagnosis.