RPM's go high low and high low when stoping at a signal
 

2005 f-150 4.6L auto
When I begin to stop at a stop sign the rpms fluctuate high(900) to low (400)
back and forth until it equals out at about around 600 or so. Don't have a rough idle, not perfect but smooth. 190000 miles. Thanks in advance.

If possible, have someone check out the charging system.

Thanks Hank