Frequent crash of pfSense 2.3.2 in last couple of weeks

[CDuv]

My v2.3.1 setup was running just fine (4.5 years now, started with v2.0.1) until August 2016 where I upgraded to v2.3.2.

Since then I got multiple crashes (pfSense detects it, reboots, and offers to send it to developers) especially 3 in the last 24h.
At first I thought that Snort was causing too much load but problem still occurs with Snort disabled.

Once pfSense did not rebooted and stopped answering pings, this was displayed on physical terminal:

4em1: discard frame w/o packet header

Here are the running services:

• darkstat (but disabled)

• dhcpd

• dpinger

• ftp-proxy (but disabled)

• ntpd

• openvpn

• sshd

• unbound

What metric could I check to see if problem is load-related?

Here is last crash report.

Setup is:
Version: 2.3.2-RELEASE (i386) built on Tue Jul 19 13:09:39 CDT 2016 FreeBSD 10.3-RELEASE-p5
CPU: Intel(R) Pentium(R) 4 CPU 2.80GHz 2 CPUs: 1 package(s) x 1 core(s) x 2 HTT threads
RAM: 3935 MiB

Thanks

[CDuv]

Forgot to say: server is a "Dell PowerEdge 750" with "em*" network cards (advertised as "Dual embedded Intel Gigabit NIC, Intel PRO/100S; Intel PRO/1000 MT; Intel
PRO/ 1000MT Dual Port" in the PDF specs).