Rough running 300CE

[Woodyreed]

Hello,
I am a newbie to the forum. I have enjoyed reading the posts and how everyone helps out each other.
Well, it's my turn to ask for help.
I have a 1993 300 CE. About two weeks ago I got a wild hair and decided to change the fuel filter. I had guy come over and replace it. Then He changed the plugs for me. Started right up and ran great.
The next day I went to pick up my son and about 7 miles into my trek I stopped at a light and the car ran really rough. I made it back home but the car was slow taking off but once it got going it ran good.
I have so far changed the injectors. I started the car and it ran great for a few seconds then it once again idled rough. Changed the plugs and the wires with no positive results. I'm getting ready to order three coil packs but I'm hesitant to do that. I feel that the problem is a simple one but I can't put my finger on it. I asked my local parts store guy about the gap for the plugs. he told me .028. is that correct? I checked the old plugs and they were at .038/
I need help!
I appreciate any and all help.
Thanks for your time