UNDER THE ANDES
by Rex Stout
The scene was not exactly new to me. Moved by the spirit of adventure, or by an access of ennui which overtakes me at times, I had several times visited the gaudy establishment of Mercer, on the fashionable side of Fifth Avenue in the Fifties. In either case I had found disappointment; where the stake is a matter of indifference there can be no excitement; and besides, I had been always in luck.
A couple of questions concerning the italicized fragments:
1. "an access of ennui": does it simply mean "ennui", or does this "access" add something to the meaning?
2. How should I understand the "in the Fifties" here?
3. "where the stake is a matter of indifference":
It's possible that "in the [Nineteen] Fifties" refers to his visits, rather than to the place's fashionability. You'd need some contextual clues to tell which (maybe outside the text - I mean, the socio-historical context. (There really should be a comma before the last three words to make this meaning clear, but I wouldn't bet against it.)