Problems with "high" state counts

[bateau]

Hi,
Sometimes, after reaching 8-9000 concurrent states, pfsense become unstable dealing with problems with packet forwarding. I am not really sure if it stops completely forwarding packets but I can say we are having problems with name resolution.
Reset states doesn't help and the only way to get it working again is a system reboot.
Generally we have 2000-5000 concurrent states with very low cpu usage and 20% mem usage.
Also bringing maximum state to 20000 doesn't help.

HW Specs:
Pentium IV 2,5Ghz
1G RAM
4 eth interfaces (2 3c905C-TX, 1 Intel PRO/1000, 1 RTL 8139)

pfsense configuration:
multiwan configuration with outgoing Failover with 2 WAN.
CARP Failover via a VLAN tagged interface
Use of aliases, NAT and firewall rules (no 1:1)
Traffic shaping is on.