What word should I use for something that fails intermittently?

Solution 1:

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

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, dodgy, or wonky mean, you'll need to follow the links. Following wonky gives you more synonyms.

Solution 2:

Try "unreliable".