Hi I have a 1998 mustang GT, It will sometimes drive great, but sometimes crank but no start, When it does start it idles weird, at stops the rpm will dip really low and then raise back up to normal. The battery in it is a little smaller than it says to get, however it does read 12.94 volts not started. at the idle it reads 14.8 , the bettry gauge on the dash will sometimes move around back and forth while idling. when i put the windows down the gauge will go down and stay where it went. any sugestions to what part i should be looking at?