The latest patch is 1.6, I believe. It comes with BI.
The patch order is 1.2 then 1.3 then 1.4 (old BI) then 1.5, then 1.6
Installing the old BI will roll your patched 1.5 RTW back, installing the new (relatively new) one will upgrade 1.5 to 1.6.
The patches fixed a lot of bugs. That might be your problem, those bugs that are not fixed.