2006 Audi A4 with rpms dropping stalling and slight shaking
 

Okay so this is my first car and i dont know very much about them but theres def something seriously wrong with my a4.
Everytime i turn on my car i give it time to warm up. But when i shift from park to reverse or park to drive the rpms drop to around 500/400 and for a split second and the engine light goes on. And at the same time this happens the car whines and stutters. Its shakes slightly and one time i put it in reverse and the moment i let go of the brake the engine turned off and i couldnt really turn The key but it still seemed to be on the windows rolled up and down and all that. After a couple minutes i tried again and the car started. This only happened once. But when i drive it the rpms are always dropping to 400 then going back to 1 plus the whining id say happen maybe every 5 minutes or so.

Also the ac works but it doesnt blow cold air ..