1999 Chevy Suburban Won't run while at idle but runs great

My '99 Chevy Suburban runs near perfectly and quietly while accelerating, but at ide it frequently dies. I had a fuel system cleaning done at the mechanics, a new distributor cap, and a new idle throtle put on. When it dies, it very gently dies. There is no sputtering or near stalling, it just gently quits. It has always started instanting with no difficulty right afterwards except for one time, the most recent, it turned over about three times before it started.

About three months ago, my check engine light came on and off for about two weeks. Then, it stayed on for about three days. They said the code was the spark plug code but said I did not need to fix it because the light shut off and has not come back on since. This may not be associated with my current stalling problem, but.