Our less than stellar experience with Macs continue as it has recently started locking up after anything between 10 minutes and an hours use.
The usual culprit for sudden locking is dodgy memory modules, so we systematically tested them all and found them to be functioning correctly. Macs are notoriously hard to self-service, which is probably a good idea for most home users, but pros find their hands tied too. However, we appear to have tripped across the solution.
The second biggest culprit after fading memory is heat. Computers generate heat, processors particularly. You could fry an egg on a processor left running under normal circumstances, so they need fans. In fact most computers have at least two and possibly four or five fans. If any one of them slow down or fail, the unit will start heating up and there comes a point when it simply cannot function and so stops processing. Hence the lock up.
For the Mac we found a small piece of software that allows manual control of the three fans sitting in it. We increased the speed of all of them and now it appears to be running fine. Problem solved, although it should never have happened in the first place.
As any technician will tell you, opening up PCs to service them usually reveals conditions suitable for smallpox to find a footing. If your unit has progressively become more unreliable it may be worth taking the side off and, very carefully, vacuuming or brushing out the internals. But do be careful, you may do harm than good if you charge in intending to get a mirror-like sheen. You're only looking to remove build up of dust.
If previous experience is anything to go by, you might find a computer under all that dust. Or money. Or some long-lost tribe.