What would cause a 1990 GMC suburban to idle up and down and cut out frequently?

A 1990 GMC Suburban idling up and down and cutting out could be caused by several issues, including a vacuum leak, which can disrupt the air-fuel mixture, or a failing idle air control valve (IAC) that regulates engine idle speed. Additionally, problems with the fuel system, such as a clogged fuel filter or failing fuel pump, may lead to inconsistent fuel delivery. Lastly, ignition system issues, such as worn spark plugs or faulty ignition coils, could also contribute to these symptoms. Proper diagnostics would be necessary to pinpoint the exact cause.