Why does my speedometer jump when I'm idling? on 2005 Chrysler Sebring

While idling, either in park or with my foot on the break my speedometer jumps like i'm going between 10-20MPH. I noticed it tends to do it more often when the heat is on. I was told it would be the speed sensors so I had those replaced and its still doing it. The first time it happened I brought it to the shop, the tranny fluid was 2 qts low and it came back with 2 codes, one for the speed sensors, the other for a gear shift sensor or something like that. Now that I've replaced the speed sensors its coming back with no codes. Please help I have no clue what is going on.

Asked by for the 2005 Chrysler Sebring
I believe the problem "IS" the alternator like one genteman stated, because when stopping at a light, my 2004 Sebring would bump and the engine would speed up, and the speedometer would show 10-20 MPH. I watched it closely and when it did it one time the battery warning light (meaning the alternator is not working) came on for about a minute and then went out. I will change the alternator tonight after work.
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Was the first code a Vehicle Speed Sensor VSS) that was initially wrong. It is only a pulse generator that puts out an alternating a/c voltage that increases with road speed. They do fail. It could be an instrument cluster problem, but it sounds like a bad ground, another circuit (brake light, heater circuit) is feeding back into the speedometer and making it pulse. You would have to look at a wiring diagram and see where the circuits cross on the power or ground side.
I guess I should have added that the engine does rev a slight amount when it does it, it's not terribly noticeable due to the fact that it only goes up to like 20mph. It does it when the heats off also, maybe its just a fluke but it tends to happen more often with it on. The other night I was going through a drive thru so my car was idling for quite a while and the check engine light came on as well as when I was driving home the RPMs were at like 3(3000 or whatever it is) and I was only going 30mph, I brought it to the shop the yesterday to have the speed sensors replaced. The check engine light hasn't come back on it but still has the problem while idling. I went back to the shop he checked for codes, it came back with nothing but said he really had no clue as to what was going on and that he would "look into it."
I have the same problem after rats got into the engine on my 2004 Sebring. Did you ever find out what the problem was on yours???
Its your alternator causing all these problems and code errors. I have a 2006 sebring touring convertible, actually love this vehicle and had no major problems until this. The engine would lunge forward at a stop while idling. speedometer would jump to 15mph while vehicle was at a stop also when more power was needed like turning on the air or letting the top down. Its about a $300 repair versus 1000's
i have the same problem and the fix for it is changing the Alternator
So did changing the alternator solve your problem? I am having the same problem. With one exception two days ago we went out to the garage and the car was dead. I've seen that before where a dying alternator can Drain the battery with diodes that are going out.