If you have an Auto zone in your area , have them check your battery and alternator (they will do it for free). My guess is that your battery is now probably bad (sulfated cells) it can not put out enough juice to keep the car running at low speed. At higher speed your alternator is putting out the power it needs to keep going, but at low speed it does not. Probably just a bad battery.
2005 Jeep Wrangler Unlimited Impact Orange
1999 Camaro SS
1969 Plymouth Road Runner
1968 Pontiac Firebird HO that needs lots of help