Hi, your problem reads differently, in that it works sometimes and not others. Original reporter can't get it to work at all ....
just an FYI, 1.10.x release is based on freebsd 10. freebsd , while having a lot of changes, also is on ipfilter 5. Why am I calling this out ? There was/is a bug in ipv6 stateful inspection , this took >2 months to figure out the issue, working with the ipfilter maintainer. It turned out to be a freebsd 10 change with packet checksums, and not an ipfilter 5 problem. It was very frustrating

pf didn't have the problem, as pf doesn't check checksums like ipfilter , which made us look at ipfilter for too long ...
There are mixed reports of ipnat issues in freebsd 10 / ipfilter combinations , but none with any real data to help track down the issue. They are mostly about ipnat having problems after several days, and a reboot fixing it, I experienced this intermittantly, but haven't since patching for the ipv6 issue several months ago.
I use t1n1wall 1.10.x at home on ALIX, behind a cable modem, and don't see these issues. I have plenty of ports forwarded, and I have nightly backups to the internet that don't fail.
What does all this mean ? I'm going to need you help to track this down and troubleshoot, and your patience