ECU fault memory readout - could be a good start. Cleaning the ISV is never a failure, cleaning all the connectors on the ISV and the throttle vavle potentiometer as well is also recommended (corrosion on the connectors is common).
Additionally, you could check for leak air on the underpressure hoses, that may cause troubles. And, plugging off the Lambda sensor is also a usual first step.
hth
Bastian
