A couple other possibilities.
- The ASE cache could be swapped out. If so, you should see heavy activity with both vmstat and iostat.
- A query that is performing an index or tablescan where previously it was a better query plan. You'll see this in the MDA tables.
I suspect swapping may be the culprit due to the waiting for regular buffer read to complete or as Avinash pointed out, it could be a SAN issue. If it is a SAN issue, I would expect high i/o waits being reflected in iostat.
hope this helps
Jason