when u start the engine when its cold, the revs under normal conditions should be roughly 1300RPM. then as the engine starts to get warmer, the revs drop down to 900RPM. this is normal.

your problem is the alternator. as you turn more electronics on, it puts too much load on the alternator. you need to replace or rebuild it. do this simple check.
-use a volt meter and measure the volage at the battery when the car is off. a good battery should indicate above 12V.
-start the engine and let the engine warm up to operating temperature. then rev the engine to 2000RPM and turn on the headlights (need 2 people).
-now check the voltage at the battery, should be roughly 13.9V to 15.1V.

let us know how you go with that.