I drove my car yesterday on the hwy for about 30 miles at about 65 to 75 mph car has been running fine with no problems, but as soon as i exit the fwy and put it on neutral it shut off on me. I came to a stop and turn it on with no problem. On the way back home i hopped on the hwy again and when i exited the same problem happened, turn on with no problem. The only thing i notice was that it was ideling i bit unsual rpm dropped to about 500rpm. what could this be?