My astra keep revving up and down ,what do think is the cause of this problem

SOURCE: What can cause revving after

A vacuum leak either from a hose or the intake rubber tube running to the throttle body. Any air that enter's the engine other than the throttle body, will cause this condition. This is the best case scenario, worst case would be an intake gasket.

SOURCE: my car keeps revving whilst sitting idle how can i fix this?

could be airflow meter fault or throttle potentiometer should see check engine light flashing if either of these sensors are bad, they can effect idle and engine performance.