PIX Accounting

Hello to the list I've configured my PIX 6.3(5) to authenticate PPTP VPN by FreeRadius. I've noticed that if a user login fails, PIX send a STOP Accounting Packet and then a START packet: in this way I have serious problem to track the simultaneous use of the users: I think it is not normal. I know...it is not a problem of freeradius...but maybe somebody of the list had the same problem in the past: do you have some suggestions? Tnx Regards Sergio Sagliocco