macbook air completely freezes

For more than 2 years, I have had a problem with my MacBook Air totally freezing. The clock stops advancing, mouse won't move, and the only way out is to hold the power button down for a full shutdown. This problem has persisted through several updates of the OS. Sometimes it happens even when no programs are running, and the computer is just sitting. It does seem to happen more often when I am running Parallels Desktop, but other than that, there is nothing consistent I have been able to discern in what causes this to happen.

Does anyone have a clue what is causing this or how to cure it?