Your tenured generation size is too small for using the CMS collector. So it appears that the CMS collector is at some time just giving up and doing a full GC colelction which can be very expensive, as you found out!
Looking at your post-GC heap size, you have about 700MB in the young gen (in a survivor space) and about 25GB in the tenured gen. Given your heap size (48G heap, 20GB young), you have only 28GB available in the tenured geneation. That gives you only 3GB overhead, or about 10%. I would not have less that 20% overhead, with 25-30% being better. If you can't increase the heap size, decrease the young genaration size, probably down to about 16GB. I also recommend that you reset the survivor space sizes (after the CMS setting). See my white paper numbered "201" at http://www.cmg.org/cgi-bin/search.cgi?q=peter+johnson&x=0&y=0. I cover the CMS collector and tuning it in that paper.