As fluteflute has said, it seems to me that questions about problems that mysteriously appear and then disappear and don't come back should be closed. These questions cannot be answered, and even if someone else had the problem, they would typically not know it was the same problem (and should post a separate question anyway).
However, I do not think these questions should be deleted (not unless there is some other reason to delete them, anyway). Problems that mysteriously happen and then stop are very useful to have documented publicly, as this can help the process of reporting and fixing bugs enormously, if someone has a similar situation or notices a pattern.
If a user asks a question about something that appears to be a bug when the question is asked, we tell them to file a bug report. But a problem that mysteriously appears and then goes away for good is not always due to a bug, so there's often no bug report. Furthermore, even if the OP did report the bug, the bug report would likely be closed if neither they nor anybody else can reproduce the bug or provide any detailed technical information about it. Later on, the problem might occur for many more users, or become more prominent and reproducible for some. At that point, having the closed question still present on AskUbuntu would provide an important resource to those people, and to the bug reporting process.
I'm not saying that sufficiently mysterious or difficult-to-reproduce bugs should ever be posted about on AskUbuntu instead of filed as bug reports. Anything someone thinks is a bug when they experience it should be filed as a bug report, and not posted here as a question (though questions about whether or not something is a bug, or how to report something as a bug, are on-topic for AskUbuntu).
This answer is adapted from a discussion in comments. Thanks to Lord of Time for pointing out that I should post my thoughts on this matter as a separate answer.