Toyota Repair: 1994 Camry Emissions, nox emissions, egr system

Question
Ok, my 1994 Camry V6 strugled last year on emissions test, this year failed
on NOx on the low speed and failed NOx on the fast speed. Everything else
passed. What is your "Guess as to the problem?"  I am guessing the catalytic
converter...but how do you TEST it to determine if it is indeed the problem?  
I had a DIamonte that did this a few years back....and no service shop could
ever diagnose the actual problem....just take your money and say it could be
this or that or a million other things. So I just had the converter replaced so I
could "force" it to pass, which it did.  But next time around, it failed.  The
emissions shop says he knows several people who have a new converter put
on each time their car is tested, just to get it to pass.   Any recommendations
or suggestions?

Answer
High NOx emissions could be cuased by other malfunctions not just the converter, NOX gases are caused by combustion temperatures,the EGR system is designed to reduce temperatures which reduce nox emissions, the egr system is not on at idle speeds, only at cruise speeds so I don't think that the egr system is the problem because it is also failing at idle, the problem may be caused by an engine that is running hotter than normal, misadjusted timing or the 3 way cat converter, before blaming the converter make sure the engine timing is correct and the engine is running at normal temperature, if so it may be time to replace the converter.