rough idle in traffic
 

hi i just bought a 2003 cummins ho with 81k it has a k&n filter on it bully dog outlook in the cab with a bullly dog torque dog underneath the hood. it idles smooth as silk in park and in neutral but when im at a stop sign the rpm needle shakes bad and so do the mirrors?. I am wondering if this could be an fca or a leaky injector or a bad injector.? i have a warranty at the dealer but im curious as to what the cause is before i drive 2 hours back to the dealership.

Thanks

torque converter just a guess