Why does your 1992 Isuzu pick up idle up and down even when you rev it up but while driving its fine. only when at idle?

Number one cause is vacuum leaks. Check for vacuum hoses broken or off. Then failed gaskets around the intake system. Intake gaskets, Plenum gasket, throttle body gasket. One way to do this is to spray carb cleaner around these gaskets and look for change in idle. Number 2 cause is failed automatic idle control valve.