I have recently found that if you have 3 or more interfaces on a 515 pix running 6.2 where the interfaces have security set to around 0, 10, 100. If you apply limits to one or two of the interfaces but not the other, when the imposed limits are reached then the unit will reset to the value in the third interface.
In my case, outside is 0, dmz is 10, inside is 100. No limit on dmz caused this problem. I apply a different limit to the dmz and after the threshold is reached as above) on the outside it resets to the value of dmz.
Unsure if this applies to 6.3 or any other 5x5 series PIX.
Cheers, Andy
Andy Simpson CNE, MCSE, CCA, CCNA