2005 Other Mazda Model S rough idle /low RPMs

Other Category problem
2005 Other Mazda Model s 4 cyl Wheel Drive Type unknown Automatic 50000 miles

I have a 2005 Mazda 3

Almost everytime I stop at a light my rpms drop to about 400 and then the car starts to shutter (like a shake) like it is going to stall but doesn't.I only notice it after the car has warmed up.

I have had the dealership look at it 3 times and they have no clue how to fix it. The computer reads nothing and the engine light does not come on. I have had the purge valve replaced. But still nothing.

Any suggestion would be great and appreciated!