idle fluctuating

I have 1996 cutlass supreme 3100 and i just had the lower intake gaskets and oil o ring replaced now when i start it up usually in the morning or after it has sitten for more than about 30 minutes it will start up and act like it wants to cut off the rpm guage fluctuates for a couple of seconds than the rpm will just raise up to 2000 come down and it will be smooth after that. it just started doing this after i changed the gaskets. Any suggestions would be appreciated thanks.

Check for a loose or missing vacuum line. Also make sure you properly torqued the intake manifold. The symptoms you describe are classic vacuum leak.