A Combination of the above. Defects in any of the above may only show up under specific circumstances. Mismatched hardware is common, even some of the large computer manufactures do insufficient analyses and testing when they select the components that make up their systems. Many components that make up a computer just don't play well together. Some modems just don't work well with specific video cards.
Some sound cards are incompatible with certain main boards.
The list goes on. Many manufactures only look at the cost of a given part, not its compatibility, thus they can often produce unstable computer systems. Hardware can become defective. Mishandling the internal components of a computer can prematurely age them, which reduces their lifespan from decades to weeks or months. Many hardware failures are subtle and will not cause the computer to completely stop working, but will cause intermittent problems.