I do have a tach and the car idles at the correct idle. When the car starts to hammer I don't notice the tach dropping in RPMs. I have taken off the EGR and cleaned it. EGR works fine.
Very strange that this only happens about a third of the time, only at a stoplight, and goes away if I put it in neutral. Otherwise, car runs beautifully.

If it is stuck piston rings or carbon build up, why wouldn't it hammer constantly?
Also, has anyone tried any additives into the crankcase to help with carbon buildup?
Thanks!