what causes motor to reve up and down when first start?

Idle control valve needs to be replaced.

Your fast idle thermo valve needs replacing. This is the part that causes the car to rev at just a slightly higher rpm upon starting. The purpose of the slightly higher rpm rev is to warm the car up in colder temperatures. After the car reaches a certain temperature, your idle air control valve takes over. Bottom line - If idle bounces immediately after starting, replace fast idle thermo valve. If idle bounces after driving a while but not when first starting, replace idle air control valve.