When my engine is still cold my RPM stays on 1500rpm for a while, when its already warmed up it goes to 1000rpm or 800rpm at idle. But when im driving around 30mph then going to a full stop i shift my gear to neutral my car is still moving while on neutral then i noticed that my rpm went down around like 300 rpm. Also when I turn off the engine then start it again after 10mins it shoots to 1000rpm then down again to 300 or 400 rpm at idle.

 

Tps