System Grinds to Halt when Posting Sales Invoices

We are still using v1.3 running on an nt 4.0 Server and 128mb, and from day 1 have found that particularly when Posting Sales Invoices and to a lesser extent whn posting purchas Invoices , the systm comes to a virtual halt. We have lived with this problem by posting when most Users are off the system . But this is no longer Practical. If any one has any suggestions , we would be very greatfull.

Are you using Navision Financials off the shelf (non customised) or has it been customised by your NSC ? if non-optimised code is extensively used or a bad use of keys has been used within the posting routines (Codeunit 12, 80 and 90) that can create performance issue that can create performance issues.