The problem here is perfectly general.  In order to construct an argument for anything that is not a simple syllogism (and perhaps even then), one must depend on the reliability of one's memory for the earlier steps and for their justifications.  Therefore, if one wants to construct anything other than the simplest of arguments for the reliability of memory, one will have to depend on the reliability of memory.  This means that any such argument is infected with a kind of circularity: one will be able to construct such an argument only by depending on the reliability of memory.  Of course, the inability to construct a noncircular argument for memory does not tend to show that memory is unreliable.  William Alston (Alston 1986) has argued convincingly that all of the basic epistemic processes share this common trait: their reliability cannot be shown without circularity. Still, we have no choice but to trust them and we shouldn't take our inability to offer a non-circular argument for them to impugn them in any way.