Spontaneous problems in Blender's 3D viewport can happen due to various reasons. It's either a setting changed unintentionally by the user due to Blender's extensive use of hotkeys or it's a momentary hickup.
Accidentally changed settings can be identified by either starting a new instance of blender or restarting blender. In both cases Blender will reload the user settings from their last saved state.
Drawing errors happen mostly due to driver problems and sometimes also on older GPUs. In my experience GPUs with shared memory sometimes do weird things once their memory limit is reached. If the problems occur often it might help playing around with the settings in "File" -> "User Preferences" -> "System".
On my old(ish) Laptop from 2009 for instance, edge select stops working after a while if I activate "Anisotropic Filtering" (it still works but the lines won't turn yellow).
If it's a general hardware-related problem, it will show up even in the default scene. If it's a memory-related glitch it most definitely will occur only after some time and often can be fixed by restarting Blender.