Carolyn,
 
The problem referred to (described) in Speak, Memory is the first of those listed in
http://www.italiascacchistica.com/a_nabokov.htm
(This is the only online reference I've found; for further discussion see
http://nnyhav.blogspot.com/2005/09/nabokovs-theme.html
 
(also parenthetically, Mark Ford's NYRB review of Pitzer et al bemused with the contention that
'He conceived of writing as a chess match with a razor-keen opponent always looking to predict his next move, and joy and triumph lay in outwitting that reader’s assumptions, and thereby stimulating “curiosity, tenderness, kindness, ecstasy.”'
when Nabokov makes it explicitly a contest between the composer and solver))