my sc300 will run good until it gets warmed up it will still run good over 1000 rpms but when im at a stoplight it will drop down to about 600 and the check engine light will come on and it will go up and down a little bit somtimes dying but as soon as i hit the gas it kinda stuters then the check engine light comes off and it runs fine i just changed plugs in it and it wasnt doing it before help me please

 

could be the fuel filter, fuel pump or maybe the fuel ecu. When was the last time you changed the filter?

 

check for vacuum leaks. you might have forgot something since you must have taken off the throttle body to do your spark plugs