On 64-bit you need to set max memory. Otherwise SQL will allocate until the OS is memory starged, and then it will get paged out (if the SQL service account doesn't have the lock pages privilege) or will cause OSm instability and even a crash ( if the SQL service account does have the lock pages privilege)

With 64GB memory, set SQL's max memory to no more than 56GB, then check that the SQL service account has permission to lock pages. You'll find that in the local security policy.

Edit: The recommendation of 56GB is assuming there's nothing other than SQL on the box. If there is, drop that value down to allow whatever else is there to run.