Some problems seem to come and go without any locateable cause. There could be any number of reasons for this computer behavior, but if your computer has some age on it, the problem could be chip creep.
When your PC is turned on, the temperature inside rises as the components generate heat. The system fans are designed to disperse this heat, but they can't completely stop parts from warming up. Warmer components are slightly bigger than colder ones because heat makes objects expand.
When your PC is turned off again, its components can cool and the temperature inside drops. Those items that had previously expanded now contract, becoming ever-so-slightly smaller. Over time, this repeated expansion and contraction can be enough to work components free from their sockets. Even if they don't become loose, the cards can develop an oxidized layer due to this movement. This oxidation inhibits conduction so the connection between component and motherboard fails, which in turn causes erratic performance or a crash.
You can fix problems caused by chip creep by removing and replacing items such as your RAM and expansion cards once a year, taking suitable anti-static precautions while doing so. By re-seating components, you improve their connections and remove any build-up of oxidation on their contacts.