Nissan Repair: 1990 Nisssan Pintara, idle on warm up, air control valve, nisssan

Question
When starting cold, the idle will be consistent for about 60 seconds, and then
start rolling (speeding up then falling back) every ten or so seconds.  When it
reaches operating temperature, the idle is consistent - what could be wrong

Answer
Arthur,

There is a temperature sensor that provides information to the computer for cold start.  Also, there is the idle air control valve that uses this information to control the idle.  If either of these is defective the idle will be inconsistant during cold start.  Think of it as a computerized choke.  The mixture has to be rich to start with, until warm.  After the car warms up the rich mode turns off and the idle air control valve is the only thing keeping the idle consistant.  Now, there is the manifold absolute pressure sensor that determines the idle value.  But, this works all the time and if it were defective the car would idle badly all the time.  I would first start with a fresh tuneup and a clean throttle body.