The biggest problem with raising the idle, especially if you're running an automatic transmission is the potential for run away(depending on the idle speed). Try running a ground wire from a GOOD chassis ground point to a GOOD metal point on the TB.
Some, in the past, have done this with very good results, as the factory ground reference has been rather questionable. They also indicated that the throttle response time was also better

 

The 500-600 is normal. For me it around 600-620 with AC on and close to 500 without AC.
Have they tried replacing the throttle body? When it stalls and you start it back up does it then idle at normal speed. Have the dealers ever tried to collect data while the problem is happening.