Try alternating the configuration of the two interfaces on the pix within the running-config, this will narrow it down to a config or hardware issue.
I have seen hundreds of problems esculating to hardware issues within the interfaces on the 515s. There are known caveats relating to certain chipsets on the interfaces.
I wouldnt go by the Cisco config checker if thats what you are using.
Andy Simpson CNE, MCSE, CCA, CCNA