Check that grounding of wire harness is clean and firm.
Check that vacuum pipeline is unbroken, twisted and in right connection.
Check that there is no obstruction in pipe.
Check that air intake pipe is not squashed or damaged.
Check that the seal between throttle body and intake manifold is perfect.
Check that ignition cable of ignition system is unbroken, no ageing and in right wiring.
Check that wires are in right connection, no loosing or poor connection for connectors.