We have had similar problems with Permount and are now trying a
synthetic product called Piccolyte, produced by Ward's. It is
inexpensive (about $30/100 ml) compared with Balsam, but seems to
work well on thicker specimens. It is a relatively new product (5 to 10
years) so I will not swear to its archival properties, but it has seemed
satisfactory over several years. Our histologists suggests ringing the
slide with nail varnish for a bit of added security.
University of Alberta