Load balanced dual-WAN causing Network Protocol Error in Firefox

[Alex Atkin UK]

I'm having a very odd random error when browsing using a dual-WAN setup.

It doesn't seem to happen on all sites, but this forum is one of them. It will occasionally tell me I lost my connection and when I try to reload I will get hangs and eventually "Network Protocol Error" after refreshing the page a few times.

I'm not seeing anything in the pfSense logs that might suggest its restarting the pf, etc. Any ideas what I should be looking for?

I do have a third WAN link that is mostly offline (its a WiFi link to my phone so I can boost download speeds without any changes to pfSense), its set to trigger by packet loss so AFAIK shouldn't be causing issues, it will just be seen as offline most of the time.