I don't know how much of your "gushing" over Spoon is hyperbole, but let's not get too far ahead of ourselves calling him "great" last week and "future HOFer" this week just because he makes a few stops behind the line of scrimmage. Seriously, someone get a mop.
What we're seeing from Spoon I think is exactly what we're supposed to be seeing. 2-3 positive impact plays is about what you should expect from a quality 4-3 OLB, regardless of whether they occur on the first 2 series of the game or the last 2.
As for why his play drops off as the game wheres on, if I had to gander a guess it would be because early in games, teams are trying to run the ball, and that remains the strength of his game, using his speed and range to defend the run. As the game wears on, it becomes more about passing the ball most weeks, and he still tends to get exposed there because his awareness is fairly average.