My magnum was going crazy with the high idling, low power and lights going off. After a few months of trying to solve the problem, all it was, was the battery. It's located in the back and if you wait til it does completely you can do a 12V 40 amp jump to the power box under the hood to get the car running again but only to pop the hatch in the back. If you have AAA they will come to your home or wherever and confirm the loss of juice of your long lived battery, (mine was original battery from '06) and replace it. Costs about $140 for the battery. There is a 3 yr warranty and free replacement should it be faulty before the 3rd years is up. After that they prorate any services needed to it. Call your AAA for correct info.
Once my new battery was installed the check engine light went off the next day and it is back to running smoothly. These cars depend a lot on a strong battery. Try this easy fix first befire your spend unnecessary money on getting a new alternator or like I did, the starter was fine as were the seals.