Suggestions for improving reliability for certain mrp ii modules?

Our company has an older mrp ii system built with the following modules: master production schedule, bill of materials, inventory and orders, materials requirements planning, and cost control. It is also tied to our general ledger and accounts receivable. The system has been pretty solid over the past few years, but since we've upgraded our computer systems the accuracy of our inventory, orders, and forecasting has been poor. How can we improve the accuracy of these modules? Should we talk to the vendor about a software update, or go for a new system altogether? Has anyone had any trouble with these systems before? Thanks.