Why does my 2001 Century stall after starting?

My car used to stall in the morning right after starting and placing into gear. It starts to chug (almost like back in the day when you had to manually choke a car) and then stalls. It would run fine the rest of the day. I took it to a mechanic who put it on the machine and said that it isn't showing any problems but they replaced the spark plugs. The issue is getting worse as the stalling is becoming more frequent. I was going to try injector cleaner but has anybody experienced this problem or have a suggestion. Thanks.