Tim,

You're probably due for a filter change.  The filter, at least on my 87,
should be easily found at any hardward store.  It's a basic AC filter
that you cut to size.  Although you can vacuum it to prolong it's life,
they're cheap so might as well buy a new one.  Besides, if yours is that
crumbly, you definately need a new one.  On my 87 it's up top and you're
staring at it when you open the back.  I image yours is in the same
place.  Only tricky part is opening it up without breaking plastic
parts.  

For the rest of you, it's good maintenance to clean or replace this
filter often (at least yearly), especially in climates where you use the
thing all the time.  It's just like the one in your house, the cleaner
it is, the less strain on your AC and the better it works.

Tonya
87 Vanagon GL Wolfs
74 S. Beetle