Have you replaced the O2 sensor?  One sending faulty signals could
account for poor performance.  The wiring to it is also known to suffer
from heat exhaustion (get it, too close to the heat from the exhaust
causing melted wires).  In theory (by the book) you are 'suppose' to
replace it about every 60k miles.  Some last a bit longer, others don't
depending on many factors.

You might also look into the vacuum system.  If the lines haven't been
replaced in the last 4 or so years, might as well, their due.  And, it's
only a couple bucks literally.  These are rubber parts, so as they heat
they expand which could be introducing small vacuum leaks at connection
points.

Just a couple ideas ...
Tonya
87 Vanagon GL Wolfs
74 S. Beetle