2002 Nissan Serena Car Shaking

I have a 2002 Nissan Sentra. Everytime I turn it on it shakes for a few minutes and is then fine. I took it to get fixed and they said it was an engine problem and fixed the gasket valve cover/4 coils and few other engine related things ($1300). However my check engine light has come back on again and the car shakes a lot when I turn it on. The mechanics said they ran every test and cannot seem to find the problem. What can be wrong with it as it shakes when I start my car but is fine once I'm driving for a few minutes.
Thanks!