Having replaced the catalytic converter last time around may have not solved the underlying issue. A smog station should have conducted a thorough diagnosis prior to condemning the last CAT. It may have been that the CAT was replaced unnecessarily, and the real culprit was a problem with the engine's air/fuel mix ratio.
It's possible that something is causing your BMW to run "hot", resulting in the creation of high NOx. The fault may be due to a lean fuel condition, high compression due to carbon build-up on the piston heads and valves; pretty much any condition which would cause the engine to run hotter than normal.
We recommend starting with a wet/dry compression check, followed by a fuel feedback test. This will ensure your BMW is in proper fuel control. Conduct a vacuum leak test next. This will show whether any unmetered air is entering the intake system; making the air/fuel mixture too lean.
Above all this, make sure you're filling up with high octane fuel. Higher octane burns cooler.