Sorry to hear your having some troubles. Here is some things to try to see if it helps out.
I'm assuming you mean the Cisco PIX. Which version? I found this on wikipedia, and might be worth trying for you...
According to a 2000 field notice, due to a "procedural error", PIX 525s with serial numbers 44480380055 through 44480480044 were manufactured with erroneous or omitted EEPROM programming in their 82559 chips that caused the onboard FastEthernet ports to behave erratically when set to full-duplex. Starting with PIX OS 5.3.1, the "eeprom update" command will reprogram the defective data and restore normal operation permanently. Viewing the field notice requires registration . Most, if not all, 525s in use today within that range have likely been corrected, but an unused or unopened unit within that range would still need the corrective action to be taken.
I would put WireShark on the 82579 side and see if the packets coming back from the PIX are there. And since the PIX is only 10/100, you should be able to put the PIX on a hub and get the data from it as well.
I'm guessing once you see what is coming and going to both sides you'll see the troubles.