performance problem with intel 82573L

Hi, has anyone experienced problem with Intel on-board 82573L cards?

I have a router with Supermicro PDSMi-LN4, p4D 3,2G,4MB cache, 1G RAM

router is doing only nat, cca 70-80Mbit avg,15k pps, only one rule for SNAT, no other rules

v. 3.0rc2

traffic load is about 25% with spikes to 50-100% and it behaves a weird, as not giving full
bandwidth to some clients, but theres plenty of bandwidth

I have found that someone had similar problem in BSD, it said something about decreased performance
because of sharing irq pin.