@wallabybob:
It appears you have some process that starts from time to time but doesn't terminate correctly, hence the available process "slots" are gradually used up. It would be useful to know which process and then determine why it isn't terminating correctly.
Please peruse the pfSense system logs looking for error reports from processes.
Do you have any packages installed?
How long does the system stay up before this happens?
Since you system seems to be able to stay up for a day (24 hours?) how about rebooting at the end of the day then in the morning ssh to it give the shell command ps ax and look for duplicate processes (some will be valid), repeat hourly and compare with previous output looking for increasing numbers of duplicate processes. This won't help it the problem is provoked by a particular "unusual" event which provokes a flurry of creation of processes which don't correctly terminate.
You could also scan the forums for topics discussing maxproc to see if anyone else has seen a similar problem.
Well, I did some more research and poking around on the router, and I've discovered the problem and fixed it. The issue was caused by NAT reflection. I have a few servers running here, and quite a few people on my LAN, all accessing local servers via their domain name, and it seems the sheer number of connections being created was causing NAT reflection to spawn off too many processes for the system to handle. I disabled NAT Reflection and am working on getting split-DNS set up now. Thank for the advice though guys. :)