If you get this message on logs:
VFR-4-FRAG_TABLE_OVERFLOW: Dialer0: the fragment table has reached its maximum threshold 16
usually indicates than you are receiving a high number of fragmented packets. It may happens using p2p applications like torrent or emule, under an fragmentation packet attack or another causes.
You can increase this value and check if warnings disappear:
router(config-if)#ip virtual-reassembly max-reassemblies 64