Having the same problem with nTop on my PFSense router.
Running PFSense 1.2.2, and nTop version 3.3.8
I've not tested it fully, but it seems to crash when the net throughput is at its highest (torrenting something at the moment so that killed it). I do have it monitoring the LAN port though, so it may be too much for it to handle.
Its running on a VIA C3 1.33GHz, with 768mb RAM, 5Gigabit ethernet ports, connected to a gigabit network, with a 20mb broadband connection with a 512k up. The network is sustaining about 8 computers, with at least 3 on constantly. Medium to high net usage. I've configured the Ethernet card that its capturing, and the password, but it keeps crashing out on me.