Is it really neccessary to change these out? I wanted to know if I should change out my thermostat and spark plug wires? I have never changed these, and I have a 105K miles on my car. I have only change spark plugs every 20-25K miles, oil changes 3-4K miles, and drain and refill coolant every 20K miles.

Also I got a question. When my car is runnin at a constant idle, the RPM gauge stays steady but I can hear the car the idle, and it sounds as if its going up and down by a 100-200 RPMs. not even idle actually, when I rev it and hold it at 2-3K RPMs, u can still kind hear it

 

105k miles, I would recommend you to change the spark plug wires, cap, rotor, check timing, timing belt, water pump, etc.

 

try cleaning out your throttle body too.