Did you analyzed your traffic?
Hello spy1309,
I think your problem is not the hardware but the traffic you have.
Did you check if one or more computers on your network are deploying any type of attack, opening so many connections, to much DNS requests, spamming , so on ?
It will be good to start other modules (spyware block, Intrusion prevention, attack blocker or spam blocker) one at the time in log mode and check what is going on there.
Or, if you are skilled with it, do a tcpdump of one interface for five minutes and then upload it on a wireshark to analize.
Regards