I have the same problem. I am running a 5 years old AMD64 box (AMD Athlon 64 X2 Dual Core Processor 4800+) on a ASUS motherboard. I have two NICs one of which is an Intel Pro 100/1000 and the other a Marvell Semiconductors Yukon 100/1000. I have not yet created any custom firewall rules. When the install was fresh I could run PFSense for a few seconds to a few minutes before Pfsense stopped serving requests. The first trouble-shooting thing I did was to turn off any equipment that might compete for DHCP addressing, but to no avail.
After realising that the Gateway-log was full of error messages such as those below I stumbled up on this topic.
Dec 30 09:16:08 apinger: Could not bind socket on address(90.226.210.126) for monitoring address 90.226.210.1(WAN_DHCP) with error Can't assign requested address
Dec 30 09:16:09 apinger: Could not bind socket on address(90.226.210.126) for monitoring address 90.226.210.1(WAN_DHCP) with error Can't assign requested address
Dec 30 09:16:10 apinger: Could not bind socket on address(90.226.210.126) for monitoring address 90.226.210.1(WAN_DHCP) with error Can't assign requested address
Dec 30 09:16:11 apinger: Could not bind socket on address(90.226.210.126) for monitoring address 90.226.210.1(WAN_DHCP) with error Can't assign requested address
Dec 30 09:16:12 apinger: Could not bind socket on address(90.226.210.126) for monitoring address 90.226.210.1(WAN_DHCP) with error Can't assign requested address
Dec 30 09:16:13 apinger: Could not bind socket on address(90.226.210.126) for monitoring address 90.226.210.1(WAN_DHCP) with error Can't assign requested address
Dec 30 09:16:14 apinger: Could not bind socket on address(90.226.210.126) for monitoring address 90.226.210.1(WAN_DHCP) with error Can't assign requested address
Dec 30 09:16:15 apinger: Could not bind socket on address(90.226.210.126) for monitoring address 90.226.210.1(WAN_DHCP) with error Can't assign requested address
Dec 30 09:16:16 apinger: Could not bind socket on address(90.226.210.126) for monitoring address 90.226.210.1(WAN_DHCP) with error Can't assign requested address
Dec 30 09:17:48 apinger: Starting Alarm Pinger, apinger(26023)
Dec 30 09:36:25 apinger: Starting Alarm Pinger, apinger(21795)
Starting out I had put the LAN Interface on the Marvell card, but that caused the Web Configurator to become unresponsive as soon as the link went down. Then I switched the interfaces (so that the WAN Interface uses the Marvell NIC) with the somewhat positive result that I could at least reboot the system remotely when the link dropped as it kept doing after a few minutes after rebooting.
After checking "Disable hardware TCP segmentation offload" in System -> Advanced, Networking tab as advised by David_W in the thread https://forum.pfsense.org/index.php?topic=96325.15, uptime increased from minutes to hours (at least in some cases, but sometimes it is minutes still).
I have now also tried the changing the System: Advanced: System Tuneable net.net.tcp.tso variable from 1 to 0 as advised by julicravo, but that made the system more unstable as changing that variable caused the system to stop serving request just a few minutes after rebooting. I must admit though that it is somewhat hard to tell if the net.net.tcp.so made any difference for the better or worse.
Right now I am running my WAN on the onboard ethernet and my LAN on the 1 GB Intel Pro NIC and this configuration works great.
I believe there is a bug in the FreeBSD Marvell driver. I will replace the Marvell card with an Intel. Will pfsense auto-detect if I switch NICs and install the proper drivers by itself?