Nissan Repair: Stalls when shifting out of park., 2006 nissan xterra, electronic throttle control

Question
Dear Sir,
I write this out of pure desperation.  I have a 2006 Nissan Xterra with the 6 cyl engine.  My dear vehicle has been in the shop almost constantly since June and we've already spent $3500 in repairs.  

It all started in June with a cracked flex plate.  This led to a damaged torque convertor.  Both were replaced and its been verified that they are the correct parts.  

After this repair (and before the vehicle came home# it was discovered that the vehicle stalled when cold and when trying to shift out of park.  This led the mechanics to discover the catalytic convertors were blown apart #exploded not melted#.  Those were replaced as well as the spark plugs. No improvement in the performance.

Since then the MAF sensor has been tested and is okay, the Electronic Throttle Control has been 'relearned' and following the directions from Nissan, the battery has been replaced, the harmonic balancer was rebuilt, a part that had been recalled was replaced- a little diode.  

The colder it is, the more likely the vehicle is to stall as you shift it out of park.  The RPM drops down to 500 as you shift out of park and frequently dies.  There is no problem with the initial start up.  The most frustrating thing is the vehicle has not thrown a check engine light except to say high idol #which led to the throttle relearn#.  The vehicle will run perfectly for several days and then stall again.  

The only symptom which is concerning is since the cats were replaced there has been an abnormal amount of white smoke/exhaust coming out the tailpipe which smells like gunpowder.  It may be a condition that led to the cats exploding.  The cause for the exhaust is yet undetermined.

I think the only other thing worth noting is the engine was replaced with a new engine at 100,000 when a mechanic failed to replace the timing chain tensioner correctly.  So at 160,000 the engine only has 60,000.  It is also worth noting that I routinely get the vehicle serviced.  Oil changes, trannie flushes, differentials, etc.  I drive 100 miles a day with a baby in the car.  I desperately need my vehicles running as well as possible.  

Anything you can suggest would be enormously helpful.  So far everyone is stumped.

Answer
hello has any body checked the head gasket as this could well be the source of the white smoke and it could be the reason for the cat to blow up although i have not heard of this before you do realize that the car can be run without the cats installed it will not do any harm in doing so but only increase the output of gasses to the environment