Please be patient with me as I learn this. Recently, my car has been running perfectly, and then all of a sudden, it decides to wreck my day. It usually revs up to 2k rpms when I start the car on a cold engine and then gradually drops to 800 rpms after 30 seconds so it can warm up and get the oil moving, right? Now when I turn it on, it goes up to 2000, then as soon as it starts to decline and reaches 1.5000, it abruptly drops to 800, after which the needle begins to jiggle up and down from 600 to 1800, and the entire vehicle and engine start shaking and rumbling!?