Idle problem

[Samlp90]

So I have problem with my idle, the car go from a 1000 rpm to 500 then goes back to a 1000 every 5-6 second! After a couple minute, then car even stop, but not every time. I have notice that when i turn on the ventillation in the car the idle stabilize to a 1000rpm and runs fine. I dont have any idea of what could be the problem, thanks for helping me.

[WDRacing]

Sounds like you have a vacuum leak somewhere. I'd start by checking all the lines and connections.

[DCaff300ZX]

Your Idle Air Control Valve (IACV) is the issue, it sounds like from the turning on the fan comment it is the FICD portion which is connected to the A/C and climate control system.
Basically the whole IACV gets sticky or clogged up, and also the connectors get corroded and make bad contact which can cause the issue you mention.
Carefully remove the connectors and clean any green corrosion, and remove the hose on the outside end of the IACV and shoot in some WD-40- a 2 second blast. If this doesn't help you may need a new one, or the trouble is elsewhere such as injectors/connectors, Coolant Temp Sensor/connector, MAF needs cleaning or corroded connector, or maybe even the PTU and it's two connectors.
Hope this helps!

[NolimitZ32]

These are all very good suggestions but you should always start by doing the easiest thing first, pull codes, just because your CEL isnt on doesnt mean there are no codes. In this case I think DCaff got it with the IACV assy.