You are already using the perfect word for unreliable software.  According to [The New Hacker's Dictionary][1]

> **flaky** /adj./ 
>
>(var sp. 'flakey') Subject to frequent lossage. This use is of course related to the common slang use of the word to describe a person as eccentric, crazy, or just unreliable. A system that is flaky is working, sort of -- enough that you are tempted to try to use it -- but fails frequently enough that the odds in favor of finishing what you start are low. Commonwealth hackish prefers dodgy or wonky.

If you want to know what [lossage][2], [dodgy][3], or [wonky][4] mean, you'll need to follow the links.  Following wonky gives you more synonyms.


  [1]: http://www.outpost9.com/reference/jargon/jargon_21.html#TAG640
  [2]: http://www.outpost9.com/reference/jargon/jargon_27.html#TAG1071
  [3]: http://www.outpost9.com/reference/jargon/jargon_19.html#TAG489
  [4]: http://www.outpost9.com/reference/jargon/jargon_38.html#TAG2001