I think it could be memory issue, it just fails with power saving modes and it can be stable under load, when full voltage applied. To check this you need to install memtester package.
For 2.4 version it would be:
fetch http://pkg.freebsd.org/freebsd:11:x86:64/latest/All/memtester-4.3.0.txz
pkg install memtester-4.3.0.txz
to run, use
memtester (size to test in MB) (loops)
memtester 512 10
Memtester for other FreeBSD version can be found here — http://portsmon.freebsd.org/portoverview.py?category=sysutils&portname=memtest
For examle 2.3 pfSense based on freebsd 10 needs this package http://pkg.freebsd.org/freebsd:10:x86:64/latest/All/memtester-4.3.0.txz
If pfSense also would not hang, then it definitely memory powersavings incompatibility issue.