A vacuum leak would be my first thought.
Double check the vaccum lines & PCV hoses are connected up properly and their connections aren't cracked or brittle. The old soapy water trick can be of use sometimes. I also have a home made smoke tester made. With the engine off, a smoker will fill the intake and vacuum system with smoke, any leaks are mostly easily spotted.
The intake gasket would be the other place a leak could easily happen. Use new ones if you can. Not too pricey.
Re-torque the intake to 23-25Nm.
Not ideal is maybe a plug is not in properly or a coil not seated well.