07 CTS Rough idle

Kenipanaque

I took my cadillac into a shop to replace the spark plugs and after that change it started to idle rough... the mechanic says that the fuel injectors are the problem but I had them changed months ago... When you first start the car shakes then run smooth and then shakes its a continuos pattern. Sometimes the car just fixes itself for a day only and the next day that rough idle is back with the check engine light. Maybe its really the fuel injectors or its the wrong spark plugs they installed but im going to a friend tomorrow to check the codes for me, Hes does not work with cadillacs that much so I just wanted to see what yall think the problem is.