Why do some outbound connections suddenly fail?

[aileron]

Hi folks!
I have a dedicated pfSense box with 4 interfaces, two LAN, two WAN. WAN is configured as failover. Further, I have a VPN connection that is routed through WAN1. LAN1 is routed through WAN1/WAN2 failover, LAN2 is only routed through WAN2. Generally, everything works like a charm.

However, there is one thing that still is bugging me. Connections from LAN1 that should be NATed through WAN1 suddenly do not work. This happens several times a day, usually the situation recovers itself - sometimes it does not, then I need to reboot the box and everything works fine again.

Let's say I try to open google.com on a LAN1 client and it runs into a timeout. From the pfSense box, however, I still can run "telnet google.com 80" which works fine. Also, if at the same time downloads are running on the very same client, they are not being interrupted. However, I cannot establish new connections. Also, traffic routed through the VPN connection (which itself is routed through WAN1) is not affected at all.

So I have absolutely no clue why at some stage connections suddenly fail. It definitely is the pfSense box that has a glitch. I also have no clue where to start debugging, I tried to enable logging on the FW rules but that did not reveal anything about this issue.

Do you folks have any idea what I should try next?