The following is taken from a book:
As a result, hosting in IIS 5/6 is notorious for instability and the frequent need to reset the server or IIS 5/6.
In the context above, why doesn't "instability" have an article, while "frequent need" does?
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The instability would point to a definite, previously defined instability.
On the other hand, omitting the article would express that hosting in IIS 5/6 just introduces all kinds of instability, or instability in general. It might show up differently all the time, the system is just instable. Therefore no definite article.
Using an indefinite article, like notorious for an instability, would point to just one kind or instance of instability that would be further defined in the text following.
So the sentence tells you:
As a result, hosting in IIS 5/6 is notorious for instability in general.
As for the need to reset, the need is defined and is a singular need, the need to reset.
All this does not depend on the conjunction. You could omit either of the two phrases with no change.